runtime/testdata/testprogcgo: fix TestCgoPanicCallback

A cgo file with "//export" declarations is not permitted to have function
definitions in the cgo comments.

Fixes #49188

Change-Id: I5c24b62b259871473ee984cea96a0edd7d42d23a
Reviewed-on: https://go-review.googlesource.com/c/go/+/359195
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: cuonglm

src/runtime/testdata/testprogcgo/panic.c

@@ -0,0 +1,9 @@
+// Copyright 2021 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.
+
+extern void panic_callback();
+
+void call_callback(void) {
+	panic_callback();
+}

src/runtime/testdata/testprogcgo/panic.go

@@ -1,16 +1,10 @@
 package main
 
-import "C"
-
 // This program will crash.
 // We want to test unwinding from a cgo callback.
 
 /*
-void panic_callback();
-
-static void call_callback(void) {
-	panic_callback();
-}
+void call_callback(void);
 */
 import "C"