[X86] Add 'sahf' to getHostCPUFeatures so -march=native will pick it up correctly.

Summary: We probably mostly get this right due to family/model/stepping mapping to CPU names. But we should detect it explicitly.

Reviewers: RKSimon, echristo, dim, spatel

Reviewed By: dim

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D43418

llvm-svn: 325439