[X86] Add 'sahf' CPU feature to frontend

Summary:
Make clang accept `-msahf` (and `-mno-sahf`) flags to activate the
`+sahf` feature for the backend, for bug 36028 (Incorrect use of
pushf/popf enables/disables interrupts on amd64 kernels).  This was
originally submitted in bug 36037 by Jonathan Looney
<jonlooney@gmail.com>.

As described there, GCC also uses `-msahf` for this feature, and the
backend already recognizes the `+sahf` feature. All that is needed is to
teach clang to pass this on to the backend.

The mapping of feature support onto CPUs may not be complete; rather, it
was chosen to match LLVM's idea of which CPUs support this feature (see
lib/Target/X86/X86.td).

I also updated the affected test case (CodeGen/attr-target-x86.c) to
match the emitted output.

Reviewers: craig.topper, coby, efriedma, rsmith

Reviewed By: craig.topper

Subscribers: emaste, cfe-commits

Differential Revision: https://reviews.llvm.org/D43394

llvm-svn: 325446