[PowerPC] Implement Vector Extract Low/High Order Builtins in LLVM/Clang

[PowerPC] Implement Vector Extract Low/High Order Builtins in LLVM/Clang

This patch implements the function prototypes vec_extractl and vec_extracth in altivec.h to utilize the vector extract double element instructions introduced in Power10.

Differential Revision: https://reviews.llvm.org/D84622