[RFC] support panic and target_has_atomic predicates

[sunshowers]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

There are a couple of new predicates in Rust 1.60: target_has_atomic
and panic. Here's a first cut at supporting them.

For now, skip implementing them in target-lexicon and always return false.

The panic predicate depends both on native platform support (default
behavior) and on a potential override through RUSTFLAGS. This can be
handled in downstream libraries that need to model such overrides.

[sunshowers]

@Jake-Shadle Having looked at it, it seems too hard to maintain support for target-lexicon -- has_atomics itself is really difficult as it's a property of the whole triple rather than that of a specific architecture 😬 For example, aarch64 has 128-bit atomics on most platforms but not on Windows or UEFI.

My suggestion would probably be to remove target-lexicon support -- it's just way too difficult to maintain support.

(As a side note, I think it would be cool to also publish the minimal set of target features in builtins.rs.)

[Jake-Shadle]

I think supporting target-lexicon is still nice since it allows this crate to support non-builtin targets to some degree, and this crate's motivating use case was supporting krates by filtering target specific dependencies from the graph, which is a more limited use case than that of eg rustc itself as most target filtering done in cargo manifests is limited to the basics. So I don't see a strong need to support every possible component that could appear in a cfg expression unless someone actually has a strong use case for it.

[sunshowers]

OK, I've changed it to just return false for target-lexicon.

[Jake-Shadle]

Thanks!