Fix building of JSC with libc++.

Follow-up to 0de22e8 and 427d50d: the former was not just a cleanup
commit, and part of the changes reverted in the latter actually broke
the build with libc++ again: clang defines __GNUC__ for compatibility
with GCC, but if libc++ is used ext/atomicity.h does not exist.

Change-Id: Id86646d38b7bc62dbe0731f8f32efb2353335210
Reviewed-by: Andy Shaw <andy.shaw@digia.com>