[PowerPC] Add asm support for cache-inhibited ld/st instructions

Add assembler support for the fixed-point cache-inhibited load/store
instructions. These are hypervisor-level only, so don't get too excited ;)

Fixes PR21650.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222976 91177308-0d34-0410-b5e6-96231b3b80d8