[PowerPC] Remove QPX/A2Q BGQ/BGP CNK support

Per RFC http://lists.llvm.org/pipermail/llvm-dev/2020-April/141295.html
no one is making use of QPX/A2Q/BGQ/BGP CNK anymore.

This patch remove the support of QPX/A2Q in llvm, BGQ/BGP in clang,
CNK support in openmp/polly.

Reviewed By: hfinkel

Differential Revision: https://reviews.llvm.org/D83915