[PowerPC] [Clang] Add vector int128 pack/unpack builtins

unsigned long long builtin_unpack_vector_int128 (vector int128_t, int);
vector int128_t builtin_pack_vector_int128 (unsigned long long, unsigned long long);

Builtins should behave the same way as in GCC.

Patch By: wuzish (Zixuan Wu)
Differential Revision: https://reviews.llvm.org/D52074

llvm-svn: 342614