This list is wrong and incomplete. My understanding is that ARCH is based on uname -p which is based on the hw.machine_arch sysctl. The list I posted in the PR was created by taking the TARGET_ARCH bits from make -C/usr/src targets (rerun for each supported release and on main). There is also a list in arch(7) (search for MACHINE_ARCH):

MACHINE_ARCH vs MACHINE_CPUARCH vs MACHINE
  MACHINE_CPUARCH should be preferred in Makefiles when the generic
  architecture is being tested.  MACHINE_ARCH should be preferred when there
  is something specific to a particular type of architecture where there is a
  choice of many, or could be a choice of many.  Use MACHINE when referring
  to the kernel, interfaces dependent on a specific type of kernel or similar
  things like boot sequences.

        MACHINE       MACHINE_CPUARCH       MACHINE_ARCH
        arm64         aarch64               aarch64
        amd64         amd64                 amd64
        arm           arm                   armv6, armv7
        i386          i386                  i386
        mips          mips                  mips, mipsel, mips64, mips64el,
                                            mipshf, mipselhf, mips64elhf,
                                            mipsn32
        powerpc       powerpc               powerpc, powerpcspe, powerpc64,
                                            powerpc64le
        riscv         riscv                 riscv64, riscv64sf

@mat Do you still plan to fix this?