Adds tls_edit utility which patches irt_core.nexe's TLS usage.

src/untrusted/irt/irt_private_tls.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 <errno.h>
 #include <stdlib.h>
 #include <unistd.h>
 
@@ skipping 19 matching lines @@
   return -NACL_SYSCALL(second_tls_set)(thread_ptr);
 }
 
 void *nacl_tls_get(void) {
 #if defined(__i386__)
   /*
    * Calling second_tls_get() works on x86-32, but reading %gs:4 is a
    * lot faster.
    */
   void *result;
-  __asm__("mov %%gs:4, %0" : "=r"(result));
+  __asm__("mov %%gs:0, %0" : "=r"(result));
   return result;
 #else
   return NACL_SYSCALL(second_tls_get)();
 #endif
 }
 
 void *__nacl_read_tp(void) {
   return nacl_tls_get();
 }