[RISCV] Add vendor-defined XTheadFMemIdx (FP Indexed Memory Operations) extension

The vendor-defined XTHeadFMemIdx (no comparable standard extension exists
at the time of writing) extension adds indexed load/store instructions
for floating-point registers.

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=f511f80fa3fcaf6bcbe727fb902b8bd5ec8f9c20

Depends on D144249

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D144647