misc/cgo/test: avoid duplicate definition with gccgo

Current versions of gccgo issue a duplicate definition error when both
a definition and an empty declaration occur. Use build tags to avoid
that case for the issue9400 subdirectory.

Change-Id: I18517af87bab05e9ca43f2f295459cf34347c317
Reviewed-on: https://go-review.googlesource.com/119896
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/test/issue9400/gccgo.go

@@ -16,6 +16,8 @@ import (
 // without writing more assembly code, which we haven't bothered to
 // do.  So this is not much of a test.
 
+var Baton int32
+
 func RewindAndSetgid() {
 	atomic.StoreInt32(&Baton, 1)
 	for atomic.LoadInt32(&Baton) != 0 {

misc/cgo/test/issue9400/stubs.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build gc
+
 package issue9400
 
 var Baton int32