Mac: Enable Mach exception handler by default

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 os
 import platform
 import re
 import subprocess
@@ skipping 1551 matching lines @@
   if env.Bit('tests_use_irt'):
     sel_ldr_flags += ['-B', nacl_env.GetIrtNexe()]
 
   if skip_bootstrap:
     loader_cmd = [loader]
   else:
     loader_cmd = env.AddBootstrap(loader, [])
 
   command = loader_cmd + sel_ldr_flags + ['--'] + command
 
-  if env.Bit('host_linux'):
+  if env.Bit('host_linux') or env.Bit('host_mac'):
     extra['using_nacl_signal_handler'] = True
 
   if env.ShouldUseVerboseOptions(extra):
     env.MakeVerboseExtraOptions(name, log_verbosity, extra)
 
   node = env.CommandTest(name, command, size, posix_path=True,
                          wrapper_program_prefix=wrapper_program_prefix, **extra)
   if env.Bit('tests_use_irt'):
     env.Alias('irt_tests', node)
   return node
@@ skipping 2090 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:')