[RISCV][MC] Add support for experimental zfa extension (FLI instruction not included)

This implements experimental support for the RISCV Zfa extension as specified here: https://github.com/riscv/riscv-isa-manual/releases/download/draft-20221119-5234c63/riscv-spec.pdf, Ch. 25. This extension has not been ratified. Once ratified, it'll move out of experimental status.

This change adds assembly support for all instructions except load-immediate instructions (fli.s/fli.d/fli.h).  Assembly support for that instruction and codegen support will follow in separate patches.

Differential Revision: https://reviews.llvm.org/D141984