[clang] [clang][X86] Fix -Wundef warning in cpuid.h (PR #89842)

[Nico Weber via cfe-commits]

nico wrote:

If your project disables warning for system headers (why even do that?), I'm guessing there's going to be a bunch of other issues you'll run into. But this change here is fine, it makes cpuid.h consisten with all the other headers in clang/lib/Headers :)

https://github.com/llvm/llvm-project/pull/89842