Switch back to panic=unwind

[geofft]

I think we need to unwind when we panic so we appropriately drop things, poison mutexes, etc. I don't really know how to integrate with the kernel's unwinder: the challenge is we need to unwind every Rust function on the stack, even if there's C code in between, but not unwind past the end of the stack. (I'm also not sure the kernel has a meaningful unwinder.) We'll need to define an appropriate / meaningful eh_personality lang item, see comments in src/libpanic_unwind/gcc.rs for details and an example.

Or, I can be convinced that not dropping things and leaving mutexes locked is sound. (Seems suboptimal though)