Don't modify memory access right when changing data in debug stub.

tests/gdb/gdb_test_guest.c

 /*
  * Copyright (c) 2012 The Native Client Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include <assert.h>
 #include <string.h>
 #include <alloca.h>
 
@@ skipping 36 matching lines @@
   int local_var = arg - 1;
   global_var = local_var;
   /*
    * Force using frame pointer for this function by calling alloca.
    * This allows to test skipping %esp modifying instructions when they
    * are located in the middle of the function.
    */
   global_ptr = alloca(arg);
 }
 
+int test_call_from_gdb(int arg) {
+  global_var = 2 * arg;
+  return 3 * arg;
+}
+
 int main(int argc, char **argv) {
   assert(argc >= 2);
 
   if (strcmp(argv[1], "break_inside_function") == 0) {
     test_two_line_function(1);
     return 0;
   }
   if (strcmp(argv[1], "stepi_after_break") == 0) {
     test_stepi_after_break();
     return 0;
   }
   if (strcmp(argv[1], "print_symbol") == 0) {
     return test_print_symbol();
   }
   if (strcmp(argv[1], "stack_trace") == 0) {
     nested_calls(1);
     return 0;
   }
   if (strcmp(argv[1], "step_from_func_start") == 0) {
     global_var = 0;
     test_step_from_function_start(2);
     return 0;
   }
+  if (strcmp(argv[1], "call_from_gdb") == 0) {
+    /* Call function so that it doesn't get optimized away. */
+    return test_call_from_gdb(0);
+  }
   return 1;
 }