abuild: downgrade binfmt-registration error to a warning

strictly speaking, it is possible for an x86_64 cpu to run 32-bit
userspace binaries without qemu emulation. it is also possible for an
aarch64 cpu to run armhf/armv7 binaries (as long as the cpu implements
it, most do). rather than check for every possible combination of when
this is allowed (host cpu + emulated target, does cpu support it, ...),
just downgrade this case to a warning, to permit non-emulated use.

ref !117 (comment 255174)