[PowerPC] Rename DarwinDirective to CPUDirective (NFC)

Summary:
This patch renames the DarwinDirective (used to identify which CPU was defined)
to CPUDirective. It also adds the getCPUDirective() method and replaces all uses
of getDarwinDirective() with getCPUDirective().

Once this patch lands and downstream users of the getDarwinDirective() method
have switched to the getCPUDirective() method, the old getDarwinDirective()
method will be removed.

Reviewers: nemanjai, hfinkel, power-llvm-team, jsji, echristo, #powerpc, jhibbits

Reviewed By: hfinkel, jsji, jhibbits

Subscribers: hiraditya, shchenz, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70352