Add -mtls-use-call option to avoid %gs:0 TLS accesses.

gcc/config/i386/nacl.opt

Index: gcc/config/i386/nacl.opt
diff --git a/gcc/config/i386/nacl.opt b/gcc/config/i386/nacl.opt
new file mode 100644
index 0000000000000000000000000000000000000000..2b915220c9683006d6a3af92c8443d4d45866f27
--- /dev/null
+++ b/gcc/config/i386/nacl.opt
@@ -0,0 +1,24 @@
+; i386-dependent options for NaCL.
+;
+; Copyright (C) 2011 Free Software Foundation, Inc.
+; Contributed by Google
+;
+; This file is part of GCC.
+;
+; GCC is free software; you can redistribute it and/or modify it under
+; the terms of the GNU General Public License as published by the Free
+; Software Foundation; either version 3, or (at your option) any later
+; version.
+;
+; GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+; WARRANTY; without even the implied warranty of MERCHANTABILITY or
+; FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+; for more details.
+;
+; You should have received a copy of the GNU General Public License
+; along with GCC; see the file COPYING3.  If not see
+; <http://www.gnu.org/licenses/>.
+
+mtls-use-call
+Target Var(TARGET_TLS_USE_CALL) Init(0)
+Use a function call rather than direct segment register access for TLS