[PowerPC] Add support for -mcpu=pwr10 in both clang and llvm

Summary:
This patch simply adds support for the new CPU in anticipation of
Power10. There isn't really any functionality added so there are no
associated test cases at this time.

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

Reviewed By: stefanp, nemanjai, amyk, #powerpc

Subscribers: NeHuang, steven.zhang, hiraditya, llvm-commits, wuzish, shchenz, cfe-commits, kbarton, echristo

Tags: #clang, #powerpc, #llvm

Differential Revision: https://reviews.llvm.org/D80020