I found a way to get a complete list.
Just need to find the right one!
(gdb) set max-completions unlimited
Note the following line has a trailing space!
(gdb) complete set architecture
set architecture A6
set architecture A7
set architecture ARC600
set architecture ARC601
set architecture ARC700
set architecture ARCv2
set architecture EM
set architecture HS
set architecture Loongarch32
set architecture Loongarch64
set architecture MSP430
set architecture MSP430X
set architecture MSP430x11x1
set architecture MSP430x12
set architecture MSP430x13
set architecture MSP430x14
set architecture MSP430x15
set architecture MSP430x16
set architecture MSP430x20
set architecture MSP430x21
set architecture MSP430x22
set architecture MSP430x23
set architecture MSP430x24
set architecture MSP430x26
set architecture MSP430x31
set architecture MSP430x32
set architecture MSP430x33
set architecture MSP430x41
set architecture MSP430x42
set architecture MSP430x43
set architecture MSP430x44
set architecture MSP430x46
set architecture MSP430x47
set architecture MSP430x54
set architecture MicroBlaze
set architecture aarch64
set architecture aarch64:armv8-r
set architecture aarch64:ilp32
set architecture aarch64:llp64
set architecture alpha
set architecture alpha:ev4
set architecture alpha:ev5
set architecture alpha:ev6
set architecture am33
set architecture am33-2
set architecture arm
set architecture arm_any
set architecture armv2
set architecture armv2a
set architecture armv3
set architecture armv3m
set architecture armv4
set architecture armv4t
set architecture armv5
set architecture armv5t
set architecture armv5te
set architecture armv5tej
set architecture armv6
set architecture armv6-m
set architecture armv6k
set architecture armv6kz
set architecture armv6s-m
set architecture armv6t2
Maybe one of these is the correct one?
set architecture armv7
set architecture armv7e-m
set architecture armv8-a
set architecture armv8-m.base
set architecture armv8-m.main
set architecture armv8-r
set architecture armv8.1-m.main
set architecture armv9-a
set architecture auto
set architecture avr
set architecture avr:1
set architecture avr:100
set architecture avr:101
set architecture avr:102
set architecture avr:103
set architecture avr:104
set architecture avr:105
set architecture avr:106
set architecture avr:107
set architecture avr:2
set architecture avr:25
set architecture avr:3
set architecture avr:31
set architecture avr:35
set architecture avr:4
set architecture avr:5
set architecture avr:51
set architecture avr:6
set architecture bfin
set architecture bpf
set architecture c5
set architecture cris
set architecture cris:common_v10_v32
set architecture crisv32
set architecture csky
set architecture csky:any
set architecture csky:ck510
set architecture csky:ck610
set architecture csky:ck801
set architecture csky:ck802
set architecture csky:ck803
set architecture csky:ck807
set architecture csky:ck810
set architecture csky:ck860
set architecture ep9312
set architecture ez80-adl
set architecture ez80-z80
set architecture fr300
set architecture fr400
set architecture fr450
set architecture fr500
set architecture fr550
set architecture frv
set architecture ft32
set architecture ft32b
set architecture gbz80
set architecture h1
set architecture h8300
set architecture h8300h
set architecture h8300hn
set architecture h8300s
set architecture h8300sn
set architecture h8300sx
set architecture h8300sxn
set architecture hppa1.0
set architecture i386
set architecture i386:intel
set architecture i386:x64-32
set architecture i386:x64-32:intel
set architecture i386:x86-64
set architecture i386:x86-64:intel
set architecture i8086
set architecture ia64-elf32
set architecture ia64-elf64
set architecture iq10
set architecture iq2000
set architecture iwmmxt
set architecture iwmmxt2
set architecture lm32
set architecture m16c
set architecture m32c
set architecture m32r
set architecture m32r2
set architecture m32rx
set architecture m68hc11
set architecture m68hc12
set architecture m68hc12:HCS12
set architecture m68k
set architecture m68k:5200
set architecture m68k:5206e
set architecture m68k:521x
set architecture m68k:5249
set architecture m68k:528x
set architecture m68k:5307
set architecture m68k:5407
set architecture m68k:547x
set architecture m68k:548x
set architecture m68k:68000
set architecture m68k:68008
set architecture m68k:68010
set architecture m68k:68020
set architecture m68k:68030
set architecture m68k:68040
set architecture m68k:68060
set architecture m68k:cfv4e
set architecture m68k:cpu32
set architecture m68k:fido
set architecture m68k:isa-a
set architecture m68k:isa-a:emac
set architecture m68k:isa-a:mac
set architecture m68k:isa-a:nodiv
set architecture m68k:isa-aplus
set architecture m68k:isa-aplus:emac
set architecture m68k:isa-aplus:mac
set architecture m68k:isa-b
set architecture m68k:isa-b:emac
set architecture m68k:isa-b:float
set architecture m68k:isa-b:float:emac
set architecture m68k:isa-b:float:mac
set architecture m68k:isa-b:mac
set architecture m68k:isa-b:nousp
set architecture m68k:isa-b:nousp:emac
set architecture m68k:isa-b:nousp:mac
set architecture m68k:isa-c
set architecture m68k:isa-c:emac
set architecture m68k:isa-c:mac
set architecture m68k:isa-c:nodiv
set architecture m68k:isa-c:nodiv:emac
set architecture m68k:isa-c:nodiv:mac
set architecture mep
set architecture mips
set architecture mips:10000
set architecture mips:12000
set architecture mips:14000
set architecture mips:16
set architecture mips:16000
set architecture mips:3000
set architecture mips:3900
set architecture mips:4000
set architecture mips:4010
set architecture mips:4100
set architecture mips:4111
set architecture mips:4120
set architecture mips:4300
set architecture mips:4400
set architecture mips:4600
set architecture mips:4650
set architecture mips:5000
set architecture mips:5400
set architecture mips:5500
set architecture mips:5900
set architecture mips:6000
set architecture mips:7000
set architecture mips:8000
set architecture mips:9000
set architecture mips:gs264e
set architecture mips:gs464
set architecture mips:gs464e
set architecture mips:interaptiv-mr2
set architecture mips:isa32
set architecture mips:isa32r2
set architecture mips:isa32r3
set architecture mips:isa32r5
set architecture mips:isa32r6
set architecture mips:isa64
set architecture mips:isa64r2
set architecture mips:isa64r3
set architecture mips:isa64r5
set architecture mips:isa64r6
set architecture mips:loongson_2e
set architecture mips:loongson_2f
set architecture mips:micromips
set architecture mips:mips5
set architecture mips
cteon
set architecture mips
cteon+
set architecture mips
cteon2
set architecture mips
cteon3
set architecture mips:sb1
set architecture mips:xlr
set architecture mn10300
set architecture moxie
set architecture msp:14
set architecture n1
set architecture n1h
set architecture n1h_v2
set architecture n1h_v3
set architecture n1h_v3m
set architecture nios2
set architecture nios2:r1
set architecture nios2:r2
set architecture or1k
set architecture or1knd
set architecture powerpc:403
set architecture powerpc:601
set architecture powerpc:603
set architecture powerpc:604
set architecture powerpc:620
set architecture powerpc:630
set architecture powerpc:7400
set architecture powerpc:750
set architecture powerpc:EC603e
set architecture powerpc:MPC8XX
set architecture powerpc:a35
set architecture powerpc:common
set architecture powerpc:common64
set architecture powerpc:e500
set architecture powerpc:e500mc
set architecture powerpc:e500mc64
set architecture powerpc:e5500
set architecture powerpc:e6500
set architecture powerpc:rs64ii
set architecture powerpc:rs64iii
set architecture powerpc:titan
set architecture powerpc:vle
set architecture r800
set architecture riscv
set architecture riscv:rv32
set architecture riscv:rv64
set architecture rl78
set architecture rs6000:6000
set architecture rs6000:rs1
set architecture rs6000:rs2
set architecture rs6000:rsc
set architecture rx
set architecture rx:v2
set architecture rx:v3
set architecture s12z
set architecture s390:31-bit
set architecture s390:64-bit
set architecture sh
set architecture sh-dsp
set architecture sh2
set architecture sh2a
set architecture sh2a-nofpu
set architecture sh2a-nofpu-or-sh3-nommu
set architecture sh2a-nofpu-or-sh4-nommu-nofpu
set architecture sh2a-or-sh3e
set architecture sh2a-or-sh4
set architecture sh2e
set architecture sh3
set architecture sh3-dsp
set architecture sh3-nommu
set architecture sh3e
set architecture sh4
set architecture sh4-nofpu
set architecture sh4-nommu-nofpu
set architecture sh4a
set architecture sh4a-nofpu
set architecture sh4al-dsp
set architecture simple
set architecture sparc
set architecture sparc:sparclet
set architecture sparc:sparclite
set architecture sparc:sparclite_le
set architecture sparc:v8plus
set architecture sparc:v8plusa
set architecture sparc:v8plusb
set architecture sparc:v8plusc
set architecture sparc:v8plusd
set architecture sparc:v8pluse
set architecture sparc:v8plusm
set architecture sparc:v8plusm8
set architecture sparc:v8plusv
set architecture sparc:v9
set architecture sparc:v9a
set architecture sparc:v9b
set architecture sparc:v9c
set architecture sparc:v9d
set architecture sparc:v9e
set architecture sparc:v9m
set architecture sparc:v9m8
set architecture sparc:v9v
set architecture tic6x
set architecture tilegx
set architecture tilegx32
set architecture tomcat
set architecture v850-rh850
set architecture v850
ld-gcc-abi
set architecture v850:rh850
set architecture v850e
set architecture v850e1
set architecture v850e1
ld-gcc-abi
set architecture v850e2
set architecture v850e2
ld-gcc-abi
set architecture v850e2v3
set architecture v850e2v3
ld-gcc-abi
set architecture v850e2v4
set architecture v850e2v4
ld-gcc-abi
set architecture v850e3v5
set architecture v850e3v5
ld-gcc-abi
set architecture v850e
ld-gcc-abi
set architecture vax
set architecture xbpf
set architecture xscale
set architecture xstormy16
set architecture xtensa
set architecture z180
set architecture z80
set architecture z80-full
set architecture z80-strict
set architecture z80n
(gdb)
