[RISCV] Add vendor-defined XTheadBb (basic bit-manipulation) extension

The vendor-defined XTHeadBb (predating the standard Zbb extension)
extension adds some bit-manipulation extensions with somewhat similar
semantics as some of the Zbb 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 XTHeadBb is
available from:
  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=8254c3d2c94ae5458095ea6c25446ba89134b9da

Depends on D143036

Differential Revision: https://reviews.llvm.org/D143439