[PowerPC] [Clang] Port MMX intrinsics and basic test cases to Power
Port mmintrin.h which include x86 MMX intrinsics implementation to PowerPC platform (using Altivec).
To make the include process correct, PowerPC's toolchain class is overrided to insert new headers directory (named ppc_wrappers) into the path. Basic test cases for several intrinsic functions are added.
The header is mainly developed by Steven Munroe, with contributions from Paul Clarke, Bill Schmidt, Jinsong Ji and Zixuan Wu.
Reviewed By: Jinsong Ji
Differential Revision: https://reviews.llvm.org/D59924