Resolve at_exit strategy after std::thread lands

[aturon]

The final step of runtime removal changes the threading/process model so that the process shuts down when the main thread exits. But several shared resources, like the helper thread for timeouts, are shut down when the main thread exits (but before the process ends), and they are not prepared to be used after shut down, but other threads may try to access them during the shutdown sequence of the main thread.

As an interim solution, the at_exit cleanup routine is simply skipped.

Ultimately, these resources should be made to safely handle asynchronous shutdown, usually by panicking if called from a detached thread when the main thread is ending.

[aturon]

cc @alexcrichton

Nominating for P-High.