clone clobbers pre-initialized stack

[stephenrkell]

Currently we assume that the new_stack passed to clone() is ours to mess with. However, it's conceivable that the clone() site has pre-populated the stack with stuff that will be used on the other side (just as we do!). So ideally we need to save that and restore it, or somesuch. Again, probably easier once we have jump-based instrumentation.