[MIPS] Restore the trusted stack in NaClSyscallSeg.

src/trusted/service_runtime/arch/mips/nacl_switch_to_app_mips.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.
  */
 
 /*
  * NaCl Service Runtime, C-level context switch code.
  */
 
 #include "native_client/src/shared/platform/nacl_check.h"
 #include "native_client/src/trusted/service_runtime/arch/mips/sel_rt.h"
 #include "native_client/src/trusted/service_runtime/nacl_app_thread.h"
 #include "native_client/src/trusted/service_runtime/nacl_globals.h"
 #include "native_client/src/trusted/service_runtime/nacl_switch_to_app.h"
 #include "native_client/src/trusted/service_runtime/sel_ldr.h"
 
 void NaClInitSwitchToApp(struct NaClApp *nap) {
  /*
   * We don't need anything here.
   */
   UNREFERENCED_PARAMETER(nap);
 }
 
 NORETURN void NaClStartThreadInApp(struct NaClAppThread *natp,
                                    uint32_t             new_prog_ctr) {
   struct NaClThreadContext  *context;
 
-  natp->user.trusted_stack_ptr = (NaClGetStackPtr() & ~0xf) + 4;
+  natp->user.trusted_stack_ptr = NaClGetStackPtr() & ~NACL_STACK_ALIGN_MASK;
 
   context = &natp->user;
   context->new_prog_ctr = new_prog_ctr;
 
   /*
    * At startup this is not the return value, but the first argument.
    * In the initial thread, it gets the pointer to the information
    * block on the stack.  Additional threads do not expect anything in
    * particular in the first argument register, so we don't bother to
    * conditionalize this.
    */
   context->sysret = context->stack_ptr;
 
   /*
-   * Just to be sure that app does not spoil gp
-   */
-  context->global_ptr = NaClGetGlobalPtr();
-
-  /*
    * context stored in $a0
    */
   NaClSwitch(context);
 }
 
 /*
  * syscall return
  */
 NORETURN void NaClSwitchToApp(struct NaClAppThread *natp) {
   NaClSwitch(&natp->user);
 }