[RISCV][CodeGen] Add codegen patterns for experimental zfa extension (try 2)

Recommit by preames with commit message, various style cleanups, and unaddressed review comments corrected.

This patch implements experimental codegen 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.

This change does not include support for FLI (upcoming in a follow up change) or FCVTMOD (not relevant for C/C++).

Differential Revision: https://reviews.llvm.org/D143982