Clean up <sys/cdefs.h> slightly.

- Remove c++0x hack from <sys/cdefs.h> that was needed when Clang did
  not fully implement C++11. We can now safely test against C++11 to
  check whether thread_local is available, like we do for all other
  C++11 keywords.

- Don't use __clang__ to test for thread safety annotation presence. It
  turns out we have a proper attribute for this.