[MLton] switching to handler caught threads

[Stephen Weeks]

> The thing I dislike about doing a canHandle-- in Prim.switchTo is that it
> will mean duplicating the logic for atomicEnd in both the backend and the
> runtime -- because we want to move to a situation where the signal handler
> is immediately invoked when canHandle reaches 0 and a signal is pending.

Yuck, I had forgotten about that.

> That seems to suggest that Prim.switchTo might actually switch to the
> signal handler.

I guess it does.