[PPC64] Add support for clrbhrb, mfbhrbe, rfebb.

This patch adds support for the ISA 2.07 additions involving the
branch history rolling buffer and event-based branching.  These will
not be used by typical applications, so built-in support is not
required.  They will only be available via inline assembly.

Assembly/disassembly tests are included in the patch.

llvm-svn: 238032