Use C-unwind ABI for __rust_start_panic in panic_abort

[ghost]

The function originally has C ABI but is called using C-unwind ABI in std:

rust/library/std/src/panicking.rs

Lines 49 to 54 in d4ad1cf

	extern "C-unwind" {
	/// `payload` is passed through another layer of raw pointers as `&mut dyn Trait` is not
	/// FFI-safe. `BoxMeUp` lazily performs allocation only when needed (this avoids allocations
	/// when using the "abort" panic runtime).
	fn __rust_start_panic(payload: *mut &mut dyn BoxMeUp) -> u32;
	}

Which might be problematic and triggers this Miri error:

error: Undefined Behavior: calling a function with ABI C using caller ABI C-unwind
   --> /home/hyd-dev/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/panicking.rs:672:9
    |
672 |         __rust_start_panic(obj)
    |         ^^^^^^^^^^^^^^^^^^^^^^^ calling a function with ABI C using caller ABI C-unwind
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

Changing the ABI of the function to C-unwind fixes the error above.

[rust-highfive]

r? @Mark-Simulacrum

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[RalfJung]

This sounds like Miri could accept it (though I am not sure of that), but I also don't think there is any reason to use a different ABI string on the two sides here. Thanks!

@bors r+ rollup

[bors]

📌 Commit 7520ea9 has been approved by RalfJung