Don't use __gxx_personality_v0 in panic_unwind on emscripten target

[hoodmane]

This resolves #85821. See also the discussion here:
emscripten-core/emscripten#17128

The consensus seems to be that rust_eh_personality is never invoked.
I patched __gxx_personality_v0 to log invocations and then ran
various panic tests and it was never called, so this analysis matches
what seems to happen in practice. This replaces the definition with
an abort, modeled on the structured exception handling implementation.

[rust-highfive]

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[joshtriplett]

@bors r+

[bors]

📌 Commit 0ec3174 has been approved by joshtriplett

[Amanieu]

@bors r-

Can you remove __gxx_personality_v0 from the extern "C" block below as well?

[Amanieu]

@bors r+

[bors]

📌 Commit 46a3f0f has been approved by Amanieu

[JohnTitor]

Failed in rollup: #97909 (comment)
@bors r-

[Amanieu]

@bors r+

[bors]

📌 Commit d2d205d has been approved by Amanieu

[Jules-Bertholet]

I've actually had a PR open for a while now (#95950) that disables unwinding for Emscripten completely, also to address the breakage in #85821. I'm glad to see that @hoodmane found an alternative solution. However as part of the discussion around #95950, the compiler team agreed not to re-enable unwinding on Emscripten without an MCP, because of the risk that a future change to Emscripten could break everything again. It doesn't look like that process was followed here. @pnkfelix thoughts?

[hoodmane]

not to re-enable unwinding ... It doesn't look like that process was followed here

This PR didn't re-enable unwinding since it was not disabled.

[hoodmane]

Probably it would be good to discuss with the emscripten team. Ideally Rust should have an error handling implementation that relies on stable interfaces from Emscripten, and the Emscripten team should maintain a stable interface that Rust can use. Also, Emscripten has two different error handling ABIs but currently Rust only supports one, in the ideal case Rust could support both.

I will open a new issue about this.