Deadlock on exit if secondary threads are writing to stdout

When using `PROXY_TO_PTHREAD` and `main()` returns (or if a secondary pthread calls `exit()`),
the `exitOnMainThread` function is used to proxy the application exit back top the main thread and
bring down the entire application.

This means that the `exit()` call ends up happens during `emscripten_current_thread_process_queued_calls`.
However, if there are threads that are printing to stdout it will then be blocked because writing
to stdout also requires a proxied operation which requires the main thread to process queued calls
but `emscripten_current_thread_process_queued_calls` does not recursion.

This means that when the main thread calls fflush(0), and it tried to `__lockfile(2)` there is
a deadlock because the thread trying to write to stdout is already holding this lock.

This issue is responsible for the flakiness in `test_pthread_c11_threads`: #14579