Calling `exit` from a pthread can result in racy behaviour/leaks

For example the new `test_emscripten_lock_busyspin_waitinf` test is currently flaky under asan/lsan due to the race conditions in the shutdown.

See https://github.com/emscripten-core/emscripten/pull/26448#issuecomment-4070883936 for more details.