[RISCV] Add vendor-defined XTHeadBa (address-generation) extension

The vendor-defined XTHeadBa (predating the standard Zba extension)
extension adds an address-generation instruction (th.addsl) with
similar semantics as sh[123]add from Zba.  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 XTHeadBa 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

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D143029