Add no-op eh_personality function to silence CI errors

[ayrtonm]

This adds a no-op implementation for the eh_personality function to silence the daily CI error messages on discord. I believe catching panics is still kinda iffy when using lld, so this shouldn't break anything in the default case.

Also I tried porting over the version from rust to the psp crate, but it's not completely trivial since it depends on the unwind crate in rust (this is different from the one on crates.io). There was also talk of moving it to a new crate in the rust PR that broke CI, so that might be a better option.

[ayrtonm]

So the build-rust and build-rust-macos tasks are failing with

error[E0152]: found duplicate lang item `eh_personality` 

because it says eh_personality is also in the panic_unwind crate. I'm not sure if it's an issue because they're not testing with a recent enough compiler or if something's being cached.

[sajattack]

Probably build-rust and build-rust-macos need a compiler update. https://github.com/overdrivenpotato/rust-psp/blob/master/ci/concourse/build-rust.yml#L14 Update the readme and cargo-psp too please with the new MSRV.

[ayrtonm]

Thanks for pointing that out, I'd just assumed everything was testing with nightly. I updated the README too.

edit: Just updated cargo-psp as well.

[ayrtonm]

Not sure how this was actually working before since the cargo-psp commit date previously matched the nightly used in CI, but I think I got the CI problem figured out.