[PowerPC] Stop defining _ARCH_PWR6X on POWER7 and up

Summary:
The predefined macro `_ARCH_PWR6X` is associated with GCC's
`-mcpu=power6x` option, which enables generation of P6 "raw mode"
instructions such as `mftgpr`.

Later POWER processors build upon the "architected mode", not the raw
one. `_ARCH_PWR6X` should not be defined for these later processors.

Fixes PR#40236.

Reviewers: echristo, hfinkel, kbarton, nemanjai, wschmidt

Reviewed By: hfinkel

Subscribers: jsji, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58128

llvm-svn: 353975