[PowerPC][Power10] Implement VSX PCV Generate Operations in LLVM/Clang

[PowerPC][Power10] Implement VSX PCV Generate Operations in LLVM/Clang

This patch implements builtins for the following prototypes for the VSX Permute
Control Vector Generate with Mask Instructions:

vector unsigned char vec_genpcvm (vector unsigned char, const int);
vector unsigned short vec_genpcvm (vector unsigned short, const int);
vector unsigned int vec_genpcvm (vector unsigned int, const int);
vector unsigned long long vec_genpcvm (vector unsigned long long, const int);

Differential Revision: https://reviews.llvm.org/D81774