runtime: check for signed zero in printfloat

Fixes #6899

R=golang-dev, r, cshapiro, iant, rsc
CC=golang-dev
https://golang.org/cl/38120043

src/pkg/runtime/print.c

@@ -236,7 +236,10 @@ runtime·printfloat(float64 v)
 	n = 7;	// digits printed
 	e = 0;	// exp
 	s = 0;	// sign
-	if(v != 0) {
+	if(v == 0) {
+		if(1/v == runtime·neginf)
+			s = 1;
+	} else {
 		// sign
 		if(v < 0) {
 			v = -v;

test/fixedbugs/issue6899.go

@@ -0,0 +1,13 @@
+// cmpout
+
+// Copyright 2013 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.
+
+package main
+
+import "math"
+
+func main() {
+	println(math.Copysign(0, -1))
+}

test/fixedbugs/issue6899.out

@@ -0,0 +1 @@
+-0.000000e+000