Tracking Issue for `naked_functions_target_rustic_abi`

The feature gate for the issue is #![feature(naked_functions_target_rustic_abi)].

Because the details of the rust abi are unstable, and a naked function must match its stated ABI, this feature gate keeps naked functions with a rustic abi ("Rust", "rust-cold", "rust-call" and "rust-intrinsic") unstable.

Splitting out this feature was decided on in https://hackmd.io/20YsMoNgRjyxhBm_aVEwQg#Stabilize-naked_functions-rust134213.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implement the RFC (cc @rust-lang/XXX -- can anyone write up mentoring
  instructions?)
• Adjust documentation (see instructions on rustc-dev-guide)
• Formatting for new syntax has been added to the Style Guide (nightly-style-procedure)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

XXX --- list all the "unresolved questions" found in the RFC to ensure they are
not forgotten

Implementation history

@rustbot label +A-naked +F-naked_functions

Start with the naked_functions_target_rustic_abi feature gate and the linked stabilization discussion to understand the required implementation and unresolved questions. Done means the RFC is implemented, documentation and Style Guide updates are complete, and a stabilization PR is accepted.