Switch nacl-clang back to gas in SCons

tests/signal_handler_single_step/nacl.scons

 # -*- python -*-
 # Copyright 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('env')
 
 # step_test_guest.nexe fails to link when Valgrind is enabled because
 # of a problem with the TLS ".tbss" section.
 if env.IsRunningUnderValgrind():
   Return()
 
 if 'TRUSTED_ENV' not in env:
   Return()
 trusted_env = env['TRUSTED_ENV']
 
 if not env.AllowInlineAssembly():
   Return()
 
 if not (env.Bit('build_x86') or env.Bit('build_arm')):
   Return()
 # This test uses POSIX signal handling so does not build on Windows.
 if env.Bit('host_windows'):
   Return()
 # This test expects that a signal handler will be able to return to
 # untrusted code, and so it does not work on Mac OS X which does not
 # restore %cs to its saved value on return from a signal handler.
 if env.Bit('host_mac'):
   Return()
-# nacl-clang's integrated assembler expands "naclcall" to a sequence which
-# separates the push of the return address from the jump, and so breaks the
-# test's expectations for the stack pointer.
-# TODO(dschuff): re-enable this test after we switch to gas with nacl-clang
-# https://code.google.com/p/nativeclient/issues/detail?id=3966
-if env.Bit('nacl_clang'):
-  Return()
 
 if env.Bit('build_x86'):
   step_test_guest = env.ComponentProgram(
       'step_test_guest', ['step_test_guest.c'],
       EXTRA_LIBS=['${NONIRT_LIBS}'],
       EXTRA_LINKFLAGS=['-nostdlib'])
 
   step_test_host = trusted_env.ComponentProgram(
       'step_test_host', ['step_test_host.c'], EXTRA_LIBS=['sel'])
 
@@ skipping 19 matching lines @@
 regs_step_test_host = trusted_env.ComponentProgram(
     'regs_step_test_host', ['regs_step_test_host.c'],
     EXTRA_LIBS=['sel', 'test_common'])
 
 node = env.CommandTest(
     'signal_handler_regs_single_step.out',
     env.AddBootstrap(regs_step_test_host, [regs_step_test_guest]))
 env.AddNodeToTestSuite(node, ['small_tests', 'nonpexe_tests'],
                        'run_signal_handler_regs_single_step_test',
                        is_broken=not env.Bit('nacl_static_link'))