cmd/internal/obj/riscv: implement floating point instructions

Add support for assembling various single-precision and double-precision
floating point instructions.

Based on the riscv-go port.

Updates #27532

Change-Id: Iac1aec9b03bb6cbf116b229daeef944d4df550fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/196839


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>