Auto merge of #129935 - RalfJung:unsupported_calling_conventions, r=compiler-errors

make unsupported_calling_conventions a hard error

This has been a future-compat lint (not shown in dependencies) since Rust 1.55, released 3 years ago. Hopefully that was enough time so this can be made a hard error now. Given that long timeframe, I think it's justified to skip the "show in dependencies" stage. There were [not many crates hitting this](rust-lang/rust#86231 (comment)) even when the lint was originally added.

This should get cratered, and I assume then it needs a t-compiler FCP. (t-compiler because this looks entirely like an implementation oversight -- for the vast majority of ABIs, we already have a hard error, but some were initially missed, and we are finally fixing that.)

Fixes rust-lang/rust#87678

Author: bors