misc/cgo/test/backdoor: add gc build tag for thunk.s

Change-Id: I6be5f610af5c56131a9d887569919372bab1d02c
Reviewed-on: https://go-review.googlesource.com/1903Reviewed-by: Minux Ma <minux@golang.org>

misc/cgo/test/backdoor/backdoor.go

@@ -4,4 +4,4 @@
 
 package backdoor
 
-func LockedOSThread() bool // in thunk.s
+func LockedOSThread() bool // in thunk.s or runtime_gccgo.c

misc/cgo/test/backdoor/runtime_gccgo.c

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // Expose some runtime functions for testing.
-// This is the gccgo version of runtime.c.
+// This is the gccgo version of thunk.s.
 
 // +build gccgo
 

misc/cgo/test/backdoor/thunk.s

@@ -5,6 +5,7 @@
 // Assembly to get into package runtime without using exported symbols.
 
 // +build amd64 amd64p32 arm 386
+// +build gc
 
 #include "textflag.h"