specify -stdlib=libc++ for darwin

It's not the default with older Apple clang builds
and without it c++11 features don't work on at least OS X 10.7:

 ./src/core/lib/gprpp/ref_counted.h:28:10: fatal error: 'atomic' file not found
 #include <atomic>
          ^~~~~~~~

I manually tested it on macOS 10.11 image and there was not a
regression.

This should fix the "Artifact Build MacOS (internal CI)" test failure.

setup.py

@@ -159,7 +159,7 @@ if EXTRA_ENV_COMPILE_ARGS is None:
   elif "linux" in sys.platform:
     EXTRA_ENV_COMPILE_ARGS += ' -std=gnu99 -fvisibility=hidden -fno-wrapv -fno-exceptions'
   elif "darwin" in sys.platform:
-    EXTRA_ENV_COMPILE_ARGS += ' -fvisibility=hidden -fno-wrapv -fno-exceptions'
+    EXTRA_ENV_COMPILE_ARGS += ' -stdlib=libc++ -fvisibility=hidden -fno-wrapv -fno-exceptions'
 EXTRA_ENV_COMPILE_ARGS += ' -DPB_FIELD_32BIT'
 
 if EXTRA_ENV_LINK_ARGS is None: