Tracking Issue for `"vectorcall"` ABI (`abi_vectorcall`)

This is a tracking issue for the "vectorcall" and "vectorcall-unwind" ABIs.
The feature gate for the issue is #![feature(abi_vectorcall)].

(Note that this feature was added in 2015, but didn't have a tracking issue until now.)

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 (#30567)
• 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

• What to do when SSE2 is missing? Currently it is a hard error to call or define a "vectorcall" function if the SSE2 target feature is missing, no matter the signature of the function (i.e., no matter whether vector registers wold actually be used for this call).
• On which targets should the ABI work? It is only defined for x86-32 and x86-64; should the ABI string even be accepted on other targets?

Implementation history

"vectorcall" added in #30567.
"vectorcall-unwind" added in #93561.

Start with the linked implementation history (#30567 and #93561), then read the stabilization guide and documentation-pr instructions referenced in the issue. The work is complete when the documentation, style-guide formatting, and stabilization steps are finished and the SSE2 and target-support questions have been resolved.