Port existing headers which include x86 intrinsics (MMX, SSE, SSE2, etc.) implementation to PowerPC platform (using Altivec).
Since x86 intrinsic headers (like mmintrin.h) are already at clang's header directory, PowerPC's toolchain class is overrided to insert new headers directory (named ppc_wrappers) into the path. Test cases for several intrinsic functions are added as an example.
Only mmintrin.h (MMX instruction set intrinsics) are added in this patch. The header, along with others, is mainly developed by Steven Munroe, with contributions from Paul Clarke, Bill Schmidt, Jinsong Ji and Zixuan Wu.