[PowerPC] [Clang] Port SSE2 intrinsics to PowerPC

Port emmintrin.h which include Intel SSE2 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.

It's a follow-up patch of D62121.

Patched by: Qiu Chaofan <qiucf@cn.ibm.com>

Differential Revision: https://reviews.llvm.org/D62569

llvm-svn: 363122