[RISCV] Add vendor-defined XTheadMemPair (two-GPR Memory Operations) extension

The vendor-defined XTHeadMemPair (no comparable standard extension exists
at the time of writing) extension adds two-GPR load/store pair 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=6e17ae625570ff8f3c12c8765b8d45d4db8694bd

Depends on D143847

Differential Revision: https://reviews.llvm.org/D144002