mark C ABI functions as nounwind

[thestinger]

It's explicitly undefined to unwind past an FFI boundary, so these functions can be considered nounwind. The soundness hole allowing safe Rust code to unwind past FFI boundaries should be fixed first (#18510).

[bstrie]

As far as I can tell, the resolution to this bug was originally implemented in #21186 , which was postponed while waiting for the decision on the RFC at rust-lang/rfcs#638 , which was eventually rejected in favor of rust-lang/rfcs#1236 , which is now implemented and stabilized. @Aatch, do you think we can make progress here now?

[Aatch]

Looks like C functions are marked nounwind now.