extending libgcc's exception delivery path

[Jason Merrill]

I think it makes sense to extend the current unwinding API to support other
representations; this will be necessary to support using the ia64 unwind
descriptors, for instance.  A Java interpreter could then provide its own
routines for unwinding.  But I think I'd prefer to hook it in at a lower
level, associating the address range for interpreted code with the Java
routines and having the unwinder look that up.

Jason