GCC_REQD and 32-bit arm

[nia <nia%NetBSD.org@localhost>]

It seems we're in a situation where no version of gcc below 10
works on 32-bit arm, resulting in mass bulk build failures whenever
something requires newer gcc on netbsd.

Does anyone have a clue about how to fix this? The logic in
gcc.mk seems to decide to use a compiler from pkgsrc after
deciding which version to use, so it's not as simple as forcing
it to always use 10 if it picks a version lower (unless we
want to force gcc10 for literally all packages).