cmd/go: add '_go_' suffix to go files compiled by gccgo to avoid naming conflicts

Fixes #8828.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/154410043

misc/cgo/test/issue8828.go

+// compile
+
+// Copyright 2014 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.
+
+// Issue 8828: compiling a file with -compiler=gccgo fails if a .c file
+// has the same name as compiled directory.
+
+package cgotest
+
+import "./issue8828"
+
+func p() {
+	issue8828.Bar()
+}

misc/cgo/test/issue8828/issue8828.c

+// Copyright 2014 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.
+
+void foo()
+{
+}

misc/cgo/test/issue8828/trivial.go

+package issue8828
+
+//void foo();
+import "C"
+
+func Bar() {
+	C.foo()
+}

src/cmd/go/build.go

@@ -1852,7 +1852,7 @@ func (gccgoToolchain) linker() string {
 }
 
 func (gccgoToolchain) gc(b *builder, p *Package, archive, obj string, importArgs []string, gofiles []string) (ofile string, output []byte, err error) {
-	out := p.Name + ".o"
+	out := "_go_.o"
 	ofile = obj + out
 	gcargs := []string{"-g"}
 	gcargs = append(gcargs, b.gccArchArgs()...)