[NaCL SDK] Add initial support for nacl-clang

native_client_sdk/src/examples/tutorial/testing/example.dsc

 {
-  'TOOLS': ['glibc', 'newlib', 'pnacl', 'linux', 'mac'],
+  'TOOLS': ['glibc', 'newlib', 'pnacl', 'linux', 'mac', 'clang-newlib'],
   'SEL_LDR': True,
   'TARGETS': [
     {
       'NAME' : 'testing',
       'TYPE' : 'main',
       'SOURCES' : ['testing.cc'],
       'LIBS' : ['ppapi_simple', 'ppapi', 'gtest', 'nacl_io', 'ppapi_cpp', 'pthread'],
       'CXXFLAGS': ['-Wno-sign-compare']
     }
   ],
   'DATA': [
     'example.js'
   ],
   'DEST': 'examples/tutorial',
   'NAME': 'testing',
   'TITLE': 'Testing with gtest',
   'GROUP': 'Tutorial'
 }