Starlarkify Apple's build flags

As part of Starlarkification, we plan to move all Apple-related build flags defined in Bazel core code into rules_apple.

This:

* gives `rules_apple` owners more complete control over Bazel's Apple support
* reduces the need for bazel / `rules_apple` release synchronization
* saves `rules_apple` contributors from having to know how to patch Bazel.
* saves Bazel devs from having to know the details of Apple

This issue tracks the effort.

Also see:

* [Starlarkify Apple flags](https://docs.google.com/document/d/1k2w0QadZUfymFtwCBtdsA6MBs0eExFZ9l08KtFQM4Es/edit?tab=t.0#heading=h.9wqxvqh3dnld)
* [Starlarkify native Bazel flags](https://docs.google.com/document/d/1yOvi4hVV7Ja32ocwVb4lsEUnijftk8nilXPncYm-BH8/edit?tab=t.0#heading=h.5mcn15i0e1ch) (proposal)
* https://github.com/bazelbuild/bazel/discussions/26521 (GitHub discussion)

Start with the linked “Starlarkify Apple flags” design document and the related GitHub discussion, then identify the Apple-related build flags currently defined in Bazel core. Done means those flags are moved into rules_apple, with the proposal’s scope and any synchronization implications addressed.

Written by the indexing model from the issue text.