add (back) unsupported_calling_conventions lint to reject more invalid calling conventions

[RalfJung]

This adds back the unsupported_calling_conventions lint that was removed in #129935, in order to start the process of dealing with #137018. Specifically, we are going for the plan laid out here:

• 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.

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[RalfJung]

@rust-lang/lang following positive vibes expressed here, I'd like to land this PR as a first step to implement the plan from that issue. Can we get this FCPd? :)

The one potential open question here is whether this should be a regular FCW first or whether it can immediately become an FCW that shows up in dependencies. I assume stdcall and fastcall are sufficiently rare that we can go for report-in-deps immediately (since they already get rejected on non-x86-32 non-Windows targets). "cdecl" might be more widely used though so it may be worth ramping up the lint more slowly.

[traviscross]

Searching across GitHub, there are 506 Rust file hits on extern "cdecl".

Regarding whether to fire the FCW in deps immediately, I lean toward it probably being OK. Given that this may show up in cfg-heavy code, I could imagine that many library authors may not notice until their downstreams report anyway. But if anyone on lang feels we should walk this more slowly, that's what I'd estimate we'd do, so I'm curious to hear what others think.

[RalfJung]

Oh that's the syntax for doing such a search... I tried and failed to get a similar result out of github search.^^