[PowerPC] [Clang] Port SSE intrinsics to PowerPC

Port xmmintrin.h which include Intel SSE intrinsics implementation to PowerPC platform (using Altivec).

The new headers containing those implemenations are located into a directory named ppc_wrappers
which has higher priority when the platform is PowerPC on Linux. They are mainly developed by Steven Munroe,
with contributions from Paul Clarke, Bill Schmidt, Jinsong Ji and Zixuan Wu.

Patched by: Qiu Chaofan <qiucf@cn.ibm.com>
Reviewed By: Jinsong Ji

Differential Revision: https://reviews.llvm.org/D62121

llvm-svn: 361928