misc/cgo/test: get rid of the backdoor package

For Go 1.5, we can use go:linkname rather than assembly thunk for gc.
Gccgo already has support for //extern.

Change-Id: I5505aa247dd5b555112f7261ed2f192c81cf0bdf
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/1888Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/test/backdoor/thunk.s → misc/cgo/test/backdoor.go

@@ -2,16 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Assembly to get into package runtime without using exported symbols.
+package cgotest
 
-// +build amd64 amd64p32 arm 386
-// +build gc
+import _ "unsafe"
 
-#include "textflag.h"
-
-#ifdef GOARCH_arm
-#define JMP B
-#endif
-
-TEXT ·LockedOSThread(SB),NOSPLIT,$0-0
-	JMP	runtime·lockedOSThread(SB)
+//go:linkname lockedOSThread runtime.lockedOSThread
+//extern runtime_lockedOSThread
+func lockedOSThread() bool

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 thunk.s or runtime_gccgo.c

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 thunk.s.
-
-// +build gccgo
-
-_Bool runtime_lockedOSThread(void);
-
-_Bool LockedOSThread(void) asm(GOPKGPATH ".LockedOSThread");
-
-_Bool
-LockedOSThread(void)
-{
-	return runtime_lockedOSThread();
-}

misc/cgo/test/callback.go

@@ -22,8 +22,6 @@ import (
 	"strings"
 	"testing"
 	"unsafe"
-
-	"./backdoor"
 )
 
 // nestedCall calls into C, back into Go, and finally to f.
@@ -50,8 +48,6 @@ func testCallbackGC(t *testing.T) {
 	nestedCall(runtime.GC)
 }
 
-var lockedOSThread = backdoor.LockedOSThread
-
 func testCallbackPanic(t *testing.T) {
 	// Make sure panic during callback unwinds properly.
 	if lockedOSThread() {