Reapply 201734 but with appropriate gcc compatibility

Because GCC incorrectly defines _mm_prefetch to take anything that casts 
to void*, people have started using that behavior.  The previous patch 
that made _mm_prefetch actually take a const char * broke compatibility 
with existing code.  This update to the patch leaves the macro that 
defines _mm_prefetch with the (void*) cast when _MSC_VER is not defined.

llvm-svn: 201901