Tracking issue for RFC 1868: A portability lint

[aturon]

This is a tracking issue for the RFC "A portability lint" (rust-lang/rfcs#1868).

Steps:

• Implement the RFC (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

Unresolved questions:

• It's not clear what the story should be for a library like libc, which currently involves intricate uses of cfg. We should have some idea for how to approach such cases before landing the RFC.

[durka]

cc #43086

[SimonSapin]

When this is implemented, reconsider ExactSizeIterator for Range<_> and RangeInclusive<_>: #43086 (comment)