cmd/compile: improve FP FMA performance on riscv64

cmd/compile: improve FP FMA performance on riscv64

FMADD/FMSUB/FNSUB are an efficient FP FMA instructions, which can
be used by the compiler to improve FP performance.

Erf               188.0n ± 2%   139.5n ± 2%  -25.82% (p=0.000 n=10)
Erfc              193.6n ± 1%   143.2n ± 1%  -26.01% (p=0.000 n=10)
Erfinv            244.4n ± 2%   172.6n ± 0%  -29.40% (p=0.000 n=10)
Erfcinv           244.7n ± 2%   173.0n ± 1%  -29.31% (p=0.000 n=10)
geomean           216.0n        156.3n       -27.65%

Ref: The RISC-V Instruction Set Manual Volume I: Unprivileged ISA
11.6 Single-Precision Floating-Point Computational Instructions

Change-Id: I89aa3a4df7576fdd47f4a6ee608ac16feafd093c
Reviewed-on: https://go-review.googlesource.com/c/go/+/506036
Reviewed-by: Joel Sing <joel@sing.id.au>
Run-TryBot: M Zhuo <mzh@golangcn.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>