Add clippy cross-checking support to CI

[RossSmyth]

I can't run this on my computer because jemallocator isn't supported on Windows. So let's see if CI is happy with this.

[RossSmyth]

My guess is missing some package

2024-02-25T19:44:42.6885970Z   configure: error: in `/home/runner/work/miri/miri/target/x86_64-apple-darwin/debug/build/jemalloc-sys-d7684c6ecce9a528/out/build':
2024-02-25T19:44:42.6887631Z   configure: error: C compiler cannot create executables
2024-02-25T19:44:42.6888730Z   See `config.log' for more details

[RalfJung]

Doesn't seem like it's possible to run clippy for targets that one can't build binaries for. Or at least, jemalloc-sys tries to do stuff in check-builds that doesn't work for cross-builds.

So we'll have to run these on their respective native hosts, I guess?

[RalfJung]

Or alternatively we could try to make jemalloc-sys an optional dependency (on by default, but turned off for clippy) and hope that this is the only dependency that causes such issues.

That seems nicer, if it works. But there might be other dependencies with similar problems.

[saethlin]

@RossSmyth We only have the jemalloc dependency to fix this issue: #3255 which we shouldn't be subject to in a check build.

So I'd support a way to toggle it off for cross-checking.

[RalfJung]

That still doesn't seem to be working

error[E0463]: can't find crate for `either`
  --> src/lib.rs:52:1
   |
52 | extern crate either; // the one from rustc
   | ^^^^^^^^^^^^^^^^^^^^ can't find crate

error[E0463]: can't find crate for `rustc_apfloat`
  --> src/lib.rs:54:1
   |
54 | extern crate rustc_apfloat;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't find crate
   |
   = help: maybe you need to install the missing components with: `rustup component add rust-src rustc-dev llvm-tools-preview`

[RalfJung]

I think we have to give up on the cross-clippy thing. We can just add a clippy run to the end of the regular CI job to ensure we have coverage.

[RossSmyth]

Yeah I've pretty much given up on it. Too many weird things to account for. I'll keep the branch up for reference.
@rustbot blocked