limit building GNU as to relevant targets

[emaste]

Summary

GNU as 2.17.50 is currently required by amd64 and i386, so build there. It was previously built but known not to be needed on 32-bit arm, so no longer built there. It does not support 64-bit arm or RISC-V.

That leaves mips, powerpc, and sparc64. For now continue building it on those targets.

[jhb]

mips doesn't use it when building with clang. If you are using external GCC for mips you will also be using external as, so I think you can actually just delete it entirely for mips. You might want to ask the powerpc folks if powerpc is in the same boat. Given that GCC 4.2.1 is dead, we should really only be enabling this for platforms that need /usr/bin/as for the in-tree LLVM toolchain which is probably just x86? Even sparc64 can go since it requires external GCC at this point.

[emaste]

@jhibbits @bdragon do we need GNU as 2.17.50 on powerpc?

[jhibbits]

@emaste nope. powerpc* don't need GNU as. LLVM has everything we need.

[emaste]

Great, thanks. I will change this to build as on i386 and amd64 only and commit that soon.

[val_packett.cool]

I've been using D18758 to get rid of gnu as on amd64 as well…