Add implementation of pthread_rwlock based on NaCl futexs

src/untrusted/pthread/pthread.gyp

 # Copyright (c) 2011 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'includes': [
     '../../../build/common.gypi',
   ],
   'variables': {
     'common_sources': [
       'nc_thread.c',
       'nc_mutex.c',
       'nc_condvar.c',
+      'nc_rwlock.c',
       'nc_semaphore.c',
       'nc_init_irt.c',
       'stack_end.c',
       '../valgrind/dynamic_annotations.c',
     ],
   },
   'targets' : [
     {
       'target_name': 'pthread_lib',
       'type': 'none',
       'variables': {
         'nlib_target': 'libpthread.a',
         'build_glibc': 0,
         'build_newlib': 1,
         'build_pnacl_newlib': 1,
       },
       'sources': ['<@(common_sources)'],
       'dependencies': [
         '<(DEPTH)/native_client/tools.gyp:prep_toolchain',
       ],
     },
   ],
 }