math: add test that covers riscv64 fnm{add,sub} codegen

math: add test that covers riscv64 fnm{add,sub} codegen

Adds a test that triggers the RISC-V fused multiply-add code
generation bug fixed by CL 506575.

Change-Id: Ia3a55a68b48c5cc6beac4e5235975dea31f3faf2
Reviewed-on: https://go-review.googlesource.com/c/go/+/507035
Auto-Submit: M Zhuo <mzh@golangcn.org>
Reviewed-by: M Zhuo <mzh@golangcn.org>
Run-TryBot: Michael Munday <mike.munday@lowrisc.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Joedian Reid <joedian@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>