Auto merge of #84158 - cratelyn:patch-extern-c-unwind-behavior, r=nikomatsakis

move new c abi abort behavior behind feature gate

*Background*

In #76570, new ABI strings including `C-unwind` were introduced. Their
behavior is specified in RFC 2945 <sup>[1]</sup>.

However, it was reported in the #ffi-unwind stream of the Rust community Zulip
that this had altered the way that `extern "C"` functions behaved even when the
`c_unwind` feature gate was not active. <sup>[2]</sup>

*Overview*

This makes a small patch to `rustc_mir_build::build::should_abort_on_panic`, so
that the same behavior from before is in place when the `c_unwind` gate is not
active.

`rustc_middle::ty::layout::fn_can_unwind` is not touched, as the visible
behavior should not differ before/after #76570. <sup>[3]</sup>

---

1: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md
2: https://rust-lang.zulipchat.com/#narrow/stream/210922-project-ffi-unwind/topic/Is.20unwinding.20through.20extern.20C.20UB.3F/near/230112325
3: https://github.com/rust-lang/rust/pull/76570/files#diff-b0320c2b8868f325d83c027fc5d71732636e9763551e35895488f30fe057c6e9L2599-R2617

[1]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md
[2]: https://rust-lang.zulipchat.com/#narrow/stream/210922-project-ffi-unwind/topic/Is.20unwinding.20through.20extern.20C.20UB.3F/near/230112325
[3]: https://github.com/rust-lang/rust/pull/76570/files#diff-b0320c2b8868f325d83c027fc5d71732636e9763551e35895488f30fe057c6e9L2599-R2617

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