[patch] Define std::promise::set_value_at_thread_exit() etc.

[Jonathan Wakely]

This adds:
  std::notify_all_at_thread_exit()
  std::promise<>::set_value_at_thread_exit()
  std::promise<>::set_exception_at_thread_exit()
  std::packaged_task<>::make_ready_at_thread_exit()

There's a linked list of callbacks that run after TLS destructors
(called by a pthread_key_create destructor) to make shared states
ready and notify condition variables.

The core of the change to futures is that the shared state is
considered ready when _M_ready == true, instead of when _M_result !=
nullptr, so that we can store a result in _M_result without making it
ready. The callback that would make it ready at thread exit stores a
weak_ptr so it can safely check whether the shared state has already
been destroyed before thread exit (see thread c++std-parallel-1162 on
the SG1 list for related discussion).

Tested x86_64-linux, I'd like to commit this next week some time.
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