ICE on double-unwind

[RalfJung]

This example demonstrates that it currently is possible to cause double-unwinding. This code actually ought to terminate in a well-defined way (as intended by WG-FFI), but variants of this example will still be possible -- by directly calling __rust_start_panic, we can circumvent the panic counter.

So there are probably two things to do here:

• Make double unwind proper UB, rather than causing an ICE
• Incorporate Guard against unwinding in cleanup code rust#92911 into Miri to make the example safely abort (there are some thoughts of doing these double unwind guards on the MIR level rather than the codegen level, in which case Miri wouldn't have to do anything special)