Rollup merge of #82686 - CDirkx:unix-platform, r=m-ou-se

Rollup merge of #82686 - CDirkx:unix-platform, r=m-ou-se

Move `std::sys::unix::platform` to `std::sys::unix::ext`

This moves the operating system dependent alias `platform` (`std::os::{linux, android, ...}`) from `std::sys::unix` to `std::sys::unix::ext` (a.k.a. `std::os::unix`), removing the need for compatibility code in `unix_ext` when documenting on another platform.

This is also a step in making it possible to properly move `std::sys::unix::ext` to `std::os::unix`, as ideally `std::sys` should not depend on the rest of `std`.