limit building GNU assembler (as) to x86
GNU as 2.17.50 is currently required by amd64 and i386 for at least one
file that cannot be assembled by Clang's integrated assembler (IAS).
Other supported CPU architectures either use Clang IAS for all assembly
files, or rely on external toolchain.
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23180