[PowerPC] [Clang] Add platform guards to PPC vector intrinsics headers

Move the platform check out of PPC Linux toolchain code and add platform guards
to the intrinsic headers, since they are supported currently only on 64-bit
PowerPC targets.

Reviewed By: Jinsong Ji

Differential Revision: https://reviews.llvm.org/D64849

llvm-svn: 367281