Add (back) `unsupported_calling_conventions` lint to reject more invalid calling conventions This adds back the `unsupported_calling_conventions` lint that was removed in https://github.com/rust-lang/rust/pull/129935, in order to start the process of dealing with https://github.com/rust-lang/rust/issues/137018. Specifically, we are going for the plan laid out [here](https://github.com/rust-lang/rust/issues/137018#issuecomment-2672118326): - thiscall, stdcall, fastcall, cdecl should only be accepted on x86-32 - vectorcall should only be accepted on x86-32 and x86-64 The difference to the status quo is that: - We stop accepting stdcall, fastcall on targets that are windows && non-x86-32 (we already don't accept these on targets that are non-windows && non-x86-32) - We stop accepting cdecl on targets that are non-x86-32 - (There is no difference for thiscall, this was already a hard error on non-x86-32) - We stop accepting vectorcall on targets that are windows && non-x86-* Vectorcall is an unstable ABI so we can just make this a hard error immediately. The others are stable, so we emit the `unsupported_calling_conventions` forward-compat lint. I set up the lint to show up in dependencies via cargo's future-compat report immediately, but we could also make it show up just for the local crate first if that is preferred. try-job: i686-msvc-1 try-job: x86_64-msvc-1 try-job: test-various