[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325439 91177308-0d34-0410-b5e6-96231b3b80d8