PNaCl Dynamic Linking: Added portable dependencies to shared objects.

tests/pnacl_dynamic_loading/test_pll_c.c

 /*
  * Copyright 2016 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.
  */
 
 extern int module_a_var;
+extern int module_b_var;
 
-int module_b_var = 1234;
+int get_module_a_var() {
+  return module_a_var;
+}
 
-int *addr_of_module_a_var = &module_a_var;
+int get_module_b_var() {
+  return module_b_var;
+}
+
+int get_module_c_var() {
+  return module_a_var - module_b_var;
+}