misc/cgo/test: fix C panic test to work with gccgo

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/14611043

misc/cgo/test/callback_c.c

@@ -64,17 +64,3 @@ callGoStackCheck(void)
 	extern void goStackCheck(void);
 	goStackCheck();
 }
-
-/* Test calling panic from C.  This is what SWIG does.  */
-
-extern void crosscall2(void (*fn)(void *, int), void *, int);
-extern void _cgo_panic(void *, int);
-
-void
-callPanic(void)
-{
-	struct { const char *p; } a;
-	a.p = "panic from C";
-	crosscall2(_cgo_panic, &a, sizeof a);
-	*(int*)1 = 1;
-}

misc/cgo/test/callback_c_gc.c

+// Copyright 2013 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.
+
+// +build gc
+
+#include "_cgo_export.h"
+
+/* Test calling panic from C.  This is what SWIG does.  */
+
+extern void crosscall2(void (*fn)(void *, int), void *, int);
+extern void _cgo_panic(void *, int);
+
+void
+callPanic(void)
+{
+	struct { const char *p; } a;
+	a.p = "panic from C";
+	crosscall2(_cgo_panic, &a, sizeof a);
+	*(int*)1 = 1;
+}

misc/cgo/test/callback_c_gccgo.c

+// Copyright 2013 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.
+
+// +build gccgo
+
+#include "_cgo_export.h"
+
+/* Test calling panic from C.  This is what SWIG does.  */
+
+extern void _cgo_panic(const char *);
+
+void
+callPanic(void)
+{
+	_cgo_panic("panic from C");
+}