misc/cgo/test: changes to pass when using gccgo

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6821067

misc/cgo/test/backdoor/runtime.c

@@ -6,6 +6,8 @@
 // Must be in a non-cgo-using package so that
 // the go command compiles this file with 6c, not gcc.
 
+// +build gc
+
 typedef char bool;
 
 bool runtime·lockedOSThread(void);

misc/cgo/test/backdoor/runtime_gccgo.c

+// Copyright 2012 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.
+
+// Expose some runtime functions for testing.
+// This is the gccgo version of runtime.c.
+
+// +build gccgo
+
+_Bool runtime_lockedOSThread(void);
+
+_Bool LockedOSThread(void) asm(GOPKGPATH ".LockedOSThread");
+
+_Bool
+LockedOSThread(void)
+{
+	return runtime_lockedOSThread();
+}