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 <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
Run-TryBot: M Zhuo <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: mengzhuo

test/codegen/math.go

@@ -57,6 +57,7 @@ func sqrt(x float64) float64 {
 	// mips64/hardfloat:"SQRTD" mips64/softfloat:-"SQRTD"
 	// wasm:"F64Sqrt"
 	// ppc64x:"FSQRT"
+	// riscv64: "FSQRTD"
 	return math.Sqrt(x)
 }
 
@@ -69,6 +70,7 @@ func sqrt32(x float32) float32 {
 	// mips64/hardfloat:"SQRTF" mips64/softfloat:-"SQRTF"
 	// wasm:"F32Sqrt"
 	// ppc64x:"FSQRTS"
+	// riscv64: "FSQRTS"
 	return float32(math.Sqrt(float64(x)))
 }