Make use of Asyncify for pthread_join

[Forked from discussion in #9579]

Rather than disallowing blocking on the main thread and giving an escape hatch in form of a setting that still allows it, we could transform `pthread_join` and friends via Asyncify instead and use `Atomics.waitAsync` under the hood.

This way the main browser thread wouldn't really be blocked, but the code would be still backwards-compatible and properly wait for thread to finish execution as user expects.