math, math/big: add support for riscv64

Based on riscv-go port.

Updates #27532

Change-Id: Id8ae7d851c393ec3702e4176c363accb0a42587f
Reviewed-on: https://go-review.googlesource.com/c/go/+/204633Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/big/arith_decl.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !math_big_pure_go
+// +build !math_big_pure_go,!riscv64
 
 package big
 

src/math/big/arith_decl_pure.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build math_big_pure_go
+// +build math_big_pure_go riscv64
 
 package big
 

src/math/stubs_riscv64.s

+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+TEXT ·Asin(SB),NOSPLIT,$0
+	JMP ·asin(SB)
+
+TEXT ·Acos(SB),NOSPLIT,$0
+	JMP ·acos(SB)
+
+TEXT ·Asinh(SB),NOSPLIT,$0
+        JMP ·asinh(SB)
+
+TEXT ·Acosh(SB),NOSPLIT,$0
+        JMP ·acosh(SB)
+
+TEXT ·Atan2(SB),NOSPLIT,$0
+	JMP ·atan2(SB)
+
+TEXT ·Atan(SB),NOSPLIT,$0
+	JMP ·atan(SB)
+
+TEXT ·Atanh(SB),NOSPLIT,$0
+	JMP ·atanh(SB)
+
+TEXT ·Min(SB),NOSPLIT,$0
+	JMP ·min(SB)
+
+TEXT ·Max(SB),NOSPLIT,$0
+	JMP ·max(SB)
+
+TEXT ·Erf(SB),NOSPLIT,$0
+	JMP ·erf(SB)
+
+TEXT ·Erfc(SB),NOSPLIT,$0
+	JMP ·erfc(SB)
+
+TEXT ·Exp2(SB),NOSPLIT,$0
+	JMP ·exp2(SB)
+
+TEXT ·Expm1(SB),NOSPLIT,$0
+	JMP ·expm1(SB)
+
+TEXT ·Exp(SB),NOSPLIT,$0
+	JMP ·exp(SB)
+
+TEXT ·Floor(SB),NOSPLIT,$0
+	JMP ·floor(SB)
+
+TEXT ·Ceil(SB),NOSPLIT,$0
+	JMP ·ceil(SB)
+
+TEXT ·Trunc(SB),NOSPLIT,$0
+	JMP ·trunc(SB)
+
+TEXT ·Frexp(SB),NOSPLIT,$0
+	JMP ·frexp(SB)
+
+TEXT ·Hypot(SB),NOSPLIT,$0
+	JMP ·hypot(SB)
+
+TEXT ·Ldexp(SB),NOSPLIT,$0
+	JMP ·ldexp(SB)
+
+TEXT ·Log10(SB),NOSPLIT,$0
+	JMP ·log10(SB)
+
+TEXT ·Log2(SB),NOSPLIT,$0
+	JMP ·log2(SB)
+
+TEXT ·Log1p(SB),NOSPLIT,$0
+	JMP ·log1p(SB)
+
+TEXT ·Log(SB),NOSPLIT,$0
+	JMP ·log(SB)
+
+TEXT ·Modf(SB),NOSPLIT,$0
+	JMP ·modf(SB)
+
+TEXT ·Mod(SB),NOSPLIT,$0
+	JMP ·mod(SB)
+
+TEXT ·Remainder(SB),NOSPLIT,$0
+	JMP ·remainder(SB)
+
+TEXT ·Sin(SB),NOSPLIT,$0
+	JMP ·sin(SB)
+
+TEXT ·Sinh(SB),NOSPLIT,$0
+	JMP ·sinh(SB)
+
+TEXT ·Cos(SB),NOSPLIT,$0
+	JMP ·cos(SB)
+
+TEXT ·Cosh(SB),NOSPLIT,$0
+	JMP ·cosh(SB)
+
+TEXT ·Sqrt(SB),NOSPLIT,$0
+	JMP ·sqrt(SB)
+
+TEXT ·Tan(SB),NOSPLIT,$0
+	JMP ·tan(SB)
+
+TEXT ·Tanh(SB),NOSPLIT,$0
+	JMP ·tanh(SB)
+
+TEXT ·Cbrt(SB),NOSPLIT,$0
+	JMP ·cbrt(SB)
+
+TEXT ·Pow(SB),NOSPLIT,$0
+	JMP ·pow(SB)