Enabling 3DNow! prefetch instruction support for a few AMD processors in the
clang front end. This change will allow the __PRFCHW__ macro to be set on these
processors and hence include prfchwintrin.h in x86intrin.h header. Support for
the intrinsic itself seems to have already been added in r178041.

Differential Revision: http://llvm-reviews.chandlerc.com/D1934