[PowerPC] Add PowerPC cmpb builtin and emit target indepedent code for XL compatibility

[PowerPC] Add PowerPC cmpb builtin and emit target indepedent code for XL compatibility

This patch is in a series of patches to provide builtins for compatibility
with the XL compiler. This patch add the builtin and emit target independent
code for __cmpb.

Reviewed By: nemanjai, #powerpc

Differential revision: https://reviews.llvm.org/D105194