Blocking a task indefinitely in the RTS

[Phil Ruffwind]

> I did try removing this check to see, but it really didn't like that. It
> caused GC to be triggered over and over again as the RTS tried desperately
> to find something to do, doesn't seem to consider "do nothing" as a valid
> state.

Oh, I see :(  I guess it's not that easy of a fix then.  Perhaps the RTS could use a new intrinsic for blocking on foreign state.