IRT: Replace __attribute__((constructor)) with explicit call in _start()

tests/irt_compatibility/nacl.scons

 # -*- python -*-
 # Copyright (c) 2013 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')
 
 # IRT backward compatibility test
 
 # This test uses a nexe built by another test.  Nexes do not get
 # staged in pexe mode, so only run this test in nonpexe mode.
 if env.Bit('pnacl_generate_pexe'):
   Return()
 
 blob_env = env['NACL_IRT_ENV'].Clone()
 blob_env.Append(LINKFLAGS=[blob_env.RodataSwitch('${IRT_BLOB_DATA_START}'),
                            blob_env.TextSwitch('${IRT_BLOB_CODE_START}')])
 
-irt_entry_obj = blob_env.ComponentObject(
-    '${MAIN_DIR}/src/untrusted/irt/irt_entry.c')
-
 # Build custom IRT library which provides a minimal subset of old
 # interface versions.
 blob_env.Append(CPPDEFINES=[['ALLOW_DYNAMIC_LINKING',
                              str(int(not env.Bit('nacl_static_link')))]])
 irt_comp_test_library = blob_env.ComponentProgram(
-    'irt_comp_test', [irt_entry_obj, 'irt_comp_interfaces.c'],
+    'irt_comp_test', ['irt_comp_interfaces.c'],
     EXTRA_LIBS=['irt_support_private'])
 
 # Don't provide IRT, we provide it ourselves via -B.
 env.ClearBits('tests_use_irt')
 
 node = env.CommandSelLdrTestNacl(
     'irt_compatibility_test.out',
     env.File('${STAGING_DIR}/hello_world.nexe'),
     sel_ldr_flags=['-B', irt_comp_test_library],
     stdout_golden=env.File('../hello_world/hello_world.stdout'))
 
 env.AddNodeToTestSuite(node, ['small_tests'], 'run_irt_compatibility_test')