Starlarkify Apple's build flags

[dabanki]

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
• Starlarkify native Bazel flags (proposal)
• Starlarkify native Bazel flags bazel#26521 (GitHub discussion)