misc/cgo/test: fix build

The last CL forgot the all-important 'backdoor' package.
Cgo-using packages compile .c files with gcc, but we want
to compile this one with 6c, so put it in a non-cgo package.

TBR=golang-dev
CC=golang-dev
https://golang.org/cl/5758063

misc/cgo/test/backdoor/backdoor.go

+// 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.
+
+package backdoor
+
+func LockedOSThread() bool  // in runtime.c

misc/cgo/test/runtime.c → misc/cgo/test/backdoor/runtime.c

@@ -3,6 +3,8 @@
 // license that can be found in the LICENSE file.
 
 // Expose some runtime functions for testing.
+// Must be in a non-cgo-using package so that
+// the go command compiles this file with 6c, not gcc.
 
 typedef char bool;
 
@@ -14,7 +16,7 @@ FLUSH(void*)
 }
 
 void
-·lockedOSThread(bool b)
+·LockedOSThread(bool b)
 {
 	b = runtime·lockedOSThread();
 	FLUSH(&b);