[PowerPC] Add clang option -m[no-]pcrel

Summary:
Add user-facing front end option to turn off pc-relative memops.
This will be compatible with gcc.

Reviewers: stefanp, nemanjai, hfinkel, power-llvm-team, #powerpc, NeHuang, saghir

Reviewed By: stefanp, NeHuang, saghir

Subscribers: saghir, wuzish, shchenz, cfe-commits, kbarton, echristo

Tags: #clang, #powerpc

Differential Revision: https://reviews.llvm.org/D80757