[MLton] Non-exhaustive exn fn match

[Matthew Fluet]

> You only have to depend on threads if you want threads.  If you
> don't use threads, you don't need the critical section.  I think we in
> fact even used to have an optimization in MLton to turn
> atomic{Begin,End} into no-ops for programs without threads.  Sadly, I
> think that optimization went by the wayside.  So, I guess you're
> right, you do have to pay some.  But the cost is only a couple of
> instructions.

It was disabled because it made asserting whether or not execution was 
currently in a critical section fail when the program failed to use 
threads.