[libc][NFC] Make cpu feature check tolerate non-x86 architectures.

The feature check should probably be enhanced for non-x86 architectures,
but this change shields them from x86 specific pieces until then.

This patch has been split out from https://reviews.llvm.org/D81533.

GitOrigin-RevId: c76a1d0fc0c148c8642397a6ec8a589256eb1dc2
1 file changed