[RISCV] Add vendor-defined XTheadCmo (Cache Management Operations) extension

The vendor-defined XTHeadCmo (there are some similarities with the
Zicbom standard extension) extension adds cache management instructions.

It is supported by the C9xx cores (e.g., found in the wild in the
Allwinner D1) by Alibaba T-Head.

The current (as of this commit) public documentation for this
extension is available at:
  https://github.com/T-head-Semi/thead-extension-spec/releases/download/2.2.2/xthead-2023-01-30-2.2.2.pdf

Support for these instructions has already landed in GNU Binutils:
  https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=a9ba8bc2d396fb8ae2b892f3bc6be8cdfe4b555c

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D144496