Auto merge of #123723 - madsmtm:apple-std-os, r=dtolnay

Make `std::os::darwin` public

I'm not sure of the reasoning behind them not being public before, but I think they should be, just like `std::os::ios` and `std::os::macos` are public.

Additionally, I've merged their source code, as it was otherwise just a bunch of unnecessary duplication.

Ultimately, I've done this PR to fix `./x build library --target=aarch64-apple-tvos,aarch64-apple-watchos,aarch64-apple-visionos`, as that currently fails because of dead code warnings.

Since you reviewed rust-lang/rust#121419
r? davidtwco

Fixes rust-lang/rust#121640.

`@rustbot` label O-tvos O-watchos O-visionos