Fix spec of Thread#status for an externally killed thread that sleeps

[iaddict]

Fixes #4705

[enebo]

On the face of this I think this seems reasonable but I think @headius should also review this since it is threading and threading is hard :)

[headius]

There could be a race here, if a thread marks itself as aborting and then some other thread tries to exit sleep at the same time, but ideally exitSleep should only be called by that thread when it transitions. The uses I reviewed seem to fit that pattern.

Worst case, some bad actor might cause an aborting thread to mark itself as runnable again. it would be no worse than now but less likely to happen.

[eregon]

FWIW The way we do in in TruffleRuby is to save/restore the Thread status around sleep/blocking tasks.
And since only the current thread modifies its status and we poll for interrupts after restoring the thread state when interrupted in a blocking task, I believe this is safe and race-free.