Fix avx builds on macosx with clang 5.0.

The macosx release of clang v5.0 identifies itself as:
Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)

This version of clang uses the older _mm_broadcastsi128_si256, like
v3.3, as given away in the LLVM svn version above.

Change-Id: I4d6d59d5454efd57d2ae9e75f5eb7486af7cbd0c