cmd/go: fix missing __mingw_fprintf symbol for cgo on windows

Fixes #5986.

R=golang-dev, rsc, alex.brainman
CC=golang-dev
https://golang.org/cl/13261055

misc/cgo/test/cgo_test.go

@@ -47,5 +47,6 @@ func TestCallbackStack(t *testing.T)       { testCallbackStack(t) }
 func TestFpVar(t *testing.T)               { testFpVar(t) }
 func Test4339(t *testing.T)                { test4339(t) }
 func Test6390(t *testing.T)                { test6390(t) }
+func Test5986(t *testing.T)                { test5986(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

misc/cgo/test/issue5986.go

+// 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 cgotest
+
+/*
+#cgo LDFLAGS: -lm
+#include <stdio.h>
+#include <math.h>
+
+static void output5986()
+{
+    int current_row = 0, row_count = 0;
+    double sum_squares = 0;
+    do {
+        if (current_row == 10) {
+            current_row = 0;
+        }
+        ++row_count;
+    }
+    while (current_row++ != 1);
+    double d =  sqrt(sum_squares / row_count);
+    printf("sqrt is: %g\n", d);
+}
+*/
+import "C"
+import "testing"
+
+func test5986(t *testing.T) {
+	C.output5986()
+}

src/cmd/go/build.go

@@ -2034,8 +2034,9 @@ func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string, gxxfile
 
 	var staticLibs []string
 	if goos == "windows" {
-		// libmingw32 and libmingwex might also use libgcc, so libgcc must come last
-		staticLibs = []string{"-lmingwex", "-lmingw32"}
+		// libmingw32 and libmingwex might also use libgcc, so libgcc must come last,
+		// and they also have some inter-dependencies, so must use linker groups.
+		staticLibs = []string{"-Wl,--start-group", "-lmingwex", "-lmingw32", "-Wl,--end-group"}
 	}
 	if cgoLibGccFile != "" {
 		staticLibs = append(staticLibs, cgoLibGccFile)