Update rust ffi code to use `C-unwind`

[stevenengler]

Most of our rust-C ffi code uses pub extern "C" fn, but these functions are not allowed by rust to panic (a panic is not allowed to cross the ffi boundary). While Shadow does do this and this is technically UB, it has seemed to work fine for us. But this may break in the future, so we should switch to pub extern "C-unwind" fn instead.

See:

• https://rust-lang.github.io/rfcs/2945-c-unwind-abi.html
• Tracking Issue for "C-unwind ABI", RFC 2945 rust-lang/rust#74990
• Partial stabilisation of c_unwind rust-lang/rust#106075
• https://github.com/rust-lang/rust/releases/tag/1.71.0

[stevenengler]

I'm going to close this since in #3210 it's good enough for now. The long-term goal is to get rid of the C code, and if a new rust version in the future causes issues we can revisit. Shadow already seems to SIGABORT when panicking so our unwinding might not be working correctly anyways.