test/codegen: add fsqrt test for riscv64

test/codegen: add fsqrt test for riscv64

Add FSQRTD FSQRTS codegen tests for riscv64

Change-Id: I16ca3753ad1ba37afbd9d0f887b078e33f98fda0
Reviewed-on: https://go-review.googlesource.com/c/go/+/503275
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Run-TryBot: M Zhuo <mzh@golangcn.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>