Allow join to be called more than once on a thread.

[AWoloszyn]

We use this because iree_thread_delete on posix calls pthread_join but we also have reasons to want to join it manually at other times.

This also reverts the previous attempt to remove the joins from deferred_work_queue.

Signed-off-by: Andrew Woloszyn [email protected]

[benvanik]

Since we don't want to carry this any longer than we have to can you add // TODO(#issue) comments to the relevant code and make sure we delete it when fixing the deferred action queue?

[benvanik]

(or ideally, move this into the deferred action queue instead?)