math/big: enable pure Go (no assembly) build with build tag

To use a pure Go implementation of the low-level arithmetic
functions (when no platform-specific assembly implementations
are available), set the build tag math_big_pure_go.

This will make it easy to vendor the math/big package where no
assembly is available (for instance for use with gc which relies
on 1.4 functionality for now).

Change-Id: I91e17c0fdc568a20ec1512d7c64621241dc60c17
Reviewed-on: https://go-review.googlesource.com/7856Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/big/arith_386.s

@@ -2,6 +2,8 @@
 // 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
+
 #include "textflag.h"
 
 // This file provides fast assembly versions for the elementary

src/math/big/arith_amd64.s

@@ -2,6 +2,8 @@
 // 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
+
 #include "textflag.h"
 
 // This file provides fast assembly versions for the elementary

src/math/big/arith_amd64p32.s

@@ -2,6 +2,8 @@
 // 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
+
 #include "textflag.h"
 
 TEXT ·mulWW(SB),NOSPLIT,$0

src/math/big/arith_arm.s

@@ -2,6 +2,8 @@
 // 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
+
 #include "textflag.h"
 
 // This file provides fast assembly versions for the elementary

src/math/big/arith_arm64.s

@@ -2,6 +2,8 @@
 // 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
+
 #include "textflag.h"
 
 // This file provides fast assembly versions for the elementary

src/math/big/arith_decl.go

@@ -2,6 +2,8 @@
 // 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
+
 package big
 
 // implemented in arith_$GOARCH.s

src/math/big/arith_decl_pure.go

+// Copyright 2015 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.
+
+// +build math_big_pure_go
+
+package big
+
+func mulWW(x, y Word) (z1, z0 Word) {
+	return mulWW_g(x, y)
+}
+
+func divWW(x1, x0, y Word) (q, r Word) {
+	return divWW_g(x1, x0, y)
+}
+
+func addVV(z, x, y []Word) (c Word) {
+	return addVV_g(z, x, y)
+}
+
+func subVV(z, x, y []Word) (c Word) {
+	return subVV_g(z, x, y)
+}
+
+func addVW(z, x []Word, y Word) (c Word) {
+	return addVW_g(z, x, y)
+}
+
+func subVW(z, x []Word, y Word) (c Word) {
+	return subVW_g(z, x, y)
+}
+
+func shlVU(z, x []Word, s uint) (c Word) {
+	return shlVU_g(z, x, s)
+}
+
+func shrVU(z, x []Word, s uint) (c Word) {
+	return shrVU_g(z, x, s)
+}
+
+func mulAddVWW(z, x []Word, y, r Word) (c Word) {
+	return mulAddVWW_g(z, x, y, r)
+}
+
+func addMulVVW(z, x []Word, y Word) (c Word) {
+	return addMulVVW_g(z, x, y)
+}
+
+func divWVW(z []Word, xn Word, x []Word, y Word) (r Word) {
+	return divWVW_g(z, xn, x, y)
+}
+
+func bitLen(x Word) (n int) {
+	return bitLen_g(x)
+}

src/math/big/arith_ppc64x.s

@@ -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 ppc64 ppc64le
+// +build !math_big_pure_go,ppc64 !math_big_pure_go,ppc64le
 
 #include "textflag.h"