Pull in r178636 from upstream llvm trunk:

  Second pass at addressing PR15351 by explicitly checking for AVX
  support when getting the host processor information.  It emits a
  .byte sequence on GNUC compilers to work around lack of xgetbv
  support with older assemblers, and resolves a comment typo found in
  the previous patch.

This should fix crashes due to emitting of AVX instructions on certain
processors, which do not support then, when using -march=native.

This is a direct commit to stable/9, since head has a complete import of
llvm/clang trunk, and there is no single commit to merge.

Reported by:	Kubilay Kocak <koobs.freebsd@gmail.com>
MFC r249011:

Fix dates in manual pages modified in 249009.
Fix mergeinfo after r249503

Submitted by:	gavin
MFC r249240:

Use pget(9) to reduce code duplication.