cmd/compile: add single-precision FMA code generation for riscv64

cmd/compile: add single-precision FMA code generation for riscv64

This CL adds FMADDS,FMSUBS,FNMADDS,FNMSUBS SSA support for riscv

Change-Id: I1e7dd322b46b9e0f4923dbba256303d69ed12066
Reviewed-on: https://go-review.googlesource.com/c/go/+/506616
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: M Zhuo <mzh@golangcn.org>