I'm trying to have the FFI exception failure support work across the board.  What I see now is that the 64-bit StackVM works, while the 64-bit CogVM does not, even though they have exactly the same exception handling machinery in place (the new squeakExceptionHandler machinery; thank you whoever wrote it, it looks very nice), and the same essential architecture for jumping back into the interpreter.

I expect the issue is that the machinery for maintaining a chain through the stack and/or the stack search used in exception delivery is broken my careless management of the C stack in the Cog VM, whereas in the StackVM the C stack remains undisturbed.

To reproduce the crash in the Cog VM and the successful failure in the StackVM in an updated trunk image simply 

- load the FFI (script attached; it does a snapshot and quit)

- run the example at the end of ExceptionInFFICallError's class comment

Any help on debugging this appreciated.