[PowerPC] Enable CR bits support for Power8 and above.

This patch turns on support for CR bit accesses for Power8 and above. The reason
why CR bits are turned on as the default for Power8 and above is that because
later architectures make use of builtins and instructions that require CR bit
accesses (such as the use of setbc in the vector string isolate predicate
and bcd builtins on Power10).

This patch also adds the clang portion to allow for turning on CR bits in the
front end if the user so desires to.

Differential Revision: https://reviews.llvm.org/D124060