Parse target_abi if it's present even though it's unstable

[carols10cents]

Hi!! 👋🏻

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

This supports parsing the currently unstable target_abi cfg predicate, which is described in:

• RFC 2992
• The associated tracking issue
• The initial implementation

Because this is unstable, the Abi will be None for all current builtins.

However, this change enables cfg-expr to successfully parse cfg predicates that contain target_abi for those crates that decide to use it.

I believe once target_abi is stabilized that this code will Just Work™️, but of course I don't have a way to predict the future 😅 I did hack the update binary to use nightly Rust and it did set the Abi field as I expected, so 🤞🏻

The reason I want this is because:

• I work on a project that uses cargo-hakari to manage a workspace hack crate.
• cargo hakari uses cfg-expr to parse cfgs.
• The project I work on uses the ahash crate.
• ahash v0.8.1 uses target_abi in some of its cfgs
• cargo hakari can't parse target_abi and fails with this error:

Error: 
   0: building package graph failed
   1: failed to construct package graph: for package 'ahash 0.8.1 (registry+https://github.com/rust-lang/crates.io-index)': for dependency 'getrandom', parsing target 'cfg(any(target_arch = "wasm32", target_abi = "unknown"))' failed: invalid cfg() expression

So from what I can tell, we either can't upgrade ahash or we can't use cargo hakari without this change to cfg-expr.

I'm totally open to being told no, this isn't something you want to support, and I'll figure out a workaround some other way :) But this seemed like the "right" place to fix it.

Related Issues

None directly related that I could find; I searched for "target_abi" and "nightly". Support older builtins is tangentially related as this is supporting all future possible predicates whereas that issue expresses desire to support all past possible ones.

There are other unstable cfg attributes like target_has_atomic_equal_alignment that ideally should be supported here too, and I'm happy to work on adding those if you'd rather support all unstable cfgs or none of them, but target_abi is the only one that's affecting me directly right now 😉

Thank you!! ❤️

[Jake-Shadle]

I believe if you rebase from main most of the lints will be fixed.