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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178636 91177308-0d34-0410-b5e6-96231b3b80d8