Actually, use GNU inline asm for cpuid with clang

Clang doesn't support the MSVC __cpuid intrinsic yet, and fixing that is
blocked on some fairly complicated issues.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188584 91177308-0d34-0410-b5e6-96231b3b80d8