[RISCV] Add vendor-defined XTHeadBs (single-bit) extension

The vendor-defined XTHeadBs (predating the standard Zbs extension)
extension adds a bit-test instruction (th.tst) with similar semantics
as bexti from Zbs.  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 XTHeadBs 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 D143394

Differential Revision: https://reviews.llvm.org/D143036