[PowerPC] Do not special case Darwin on PowerPC in target cpu handling

Summary: This patch removes the special handling for Darwin on PowerPC in the default target cpu handling, because Darwin is no longer supported on the PowerPC platform.

Reviewers: hubert.reinterpretcast, daltenty

Reviewed By: hubert.reinterpretcast

Subscribers: wuzish, nemanjai, shchenz, steven.zhang, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81115