[RISCV][MC] Add FLI instruction support for the experimental zfa extension

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 load-immediate instructions (fli.s/fli.d/fli.h). The assembly prefers decimal constants in C-like syntax. In my implementation, an integer encoding ranging from 0 to 31 can also be accepted, but for the MCInst printer, the constant is specified in decimal notation by default.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D140460