[libc++] Fix CI and Linux failures after landing D68480

[libc++] Fix CI and Linux failures after landing D68480

- Avoid using C++11-and-later features in <atomic>:
  Historically, we've supported <atomic> in C++03, so we can't use C++11
  features in that header. This is something we really need to change,
  since our implementation of <atomic> is starting to accumulate technical
  debt because of that.
- Mark a test as unsupported on single threaded systems
- Add missing symbols to the Linux ABI list