arm: fail early on gcc builds

Since at least 2022 (see https://reviews.freebsd.org/D36754), it has
not been possible to build armv6/armv7 with gcc due to atomics macros
gcc doesn't like. Prevent developers doing due diligance from wasting
time and CPU cycles on this combination as it just fails to build in
libc.

Reviewed by: imp, andrew
Differential Revision: https://reviews.freebsd.org/D45193