Tracking Issue for `c_variadic_experimental_arch`

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

Allows defining c-variadic functions on targets where this feature has not yet undergone sufficient testing for stabilization.

Currently this feature covers

• avr
• sparc (note that sparc64 does support c-variadic definitions)
• m64k
• msp430

Defining c-variadic functions using a custom architecture (e.g. via a custom target.json) is also gated by this feature.

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
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• list all the "unresolved questions" found in the RFC to ensure they are
  not forgotten.

Implementation history

Start with the c_variadic_experimental_arch feature gate and the listed target architectures: avr, sparc, m64k, and msp430, including custom targets. Use the linked rustc-dev-guide stabilization and documentation instructions to identify the required work. Done means the implementation, documentation, stabilization PR, and unresolved questions listed in the tracking checklist are addressed.