Investigate and implement a scheme to consistently apply lints for all compiler crates

[jieyouxu]

... Especially for build scripts or proc-macro crates.

There's caveats like:

• RUSTFLAGS is only applied to target crates, not to host crates.
• cargo's current [lints] only supports "regular" lints not tool lints like rustc::xxx.

One possible scheme may be to...

[..] unconditionally passing an arbitrary flag and asserting its presence in each compiler crate. Ugly, but it could work I think.
#138035 (comment)

Discussion: https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Consistent.20handling.20of.20compiler.2F*.20RUSTFLAGs.20and.20lints

[nnethercote]

And #138035 was a hacky initial attempt at this. Probably not appropriate to merge, but a good reference.