Fix two register leaks in the trusted->untrusted context switch

SConstruct

 #! -*- python -*-
 # 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.
 
 import atexit
 import glob
 import os
 import platform
 import shutil
@@ skipping 3465 matching lines @@
     'tests/sbrk/nacl.scons',
     'tests/sel_main_chrome/nacl.scons',
     'tests/signal_handler/nacl.scons',
     'tests/sleep/nacl.scons',
     'tests/srpc/nacl.scons',
     'tests/srpc_hw/nacl.scons',
     'tests/srpc_message/nacl.scons',
     'tests/stack_alignment/nacl.scons',
     'tests/stubout_mode/nacl.scons',
     'tests/sysbasic/nacl.scons',
+    'tests/syscall_return_regs/nacl.scons',
     'tests/syscall_return_sandboxing/nacl.scons',
     'tests/syscalls/nacl.scons',
     'tests/threads/nacl.scons',
     'tests/time/nacl.scons',
     'tests/tls/nacl.scons',
     'tests/tls_perf/nacl.scons',
     'tests/tls_segment_x86_32/nacl.scons',
     'tests/toolchain/nacl.scons',
     'tests/toolchain/arm/nacl.scons',
     'tests/unittests/shared/platform/nacl.scons',
@@ skipping 549 matching lines @@
   nacl_env.ValidateSdk()
 
 if BROKEN_TEST_COUNT > 0:
   msg = "There are %d broken tests." % BROKEN_TEST_COUNT
   if GetOption('brief_comstr'):
     msg += " Add --verbose to the command line for more information."
   print msg
 
 # separate warnings from actual build output
 Banner('B U I L D - O U T P U T:')