Cleanup Apple-related code in rustc_target
While working on https://github.com/rust-lang/rust/pull/103455, the consistency of the `rustc_target` code for Apple's platforms was "kind of bad." There were two "base" files (`apple_base.rs` and `apple_sdk_base.rs`) that the targets each pulled some parts out of, each and all of them were written slightly differently, and sometimes missed comments other implementations had.
So to hopefully make future maintenance, like implementing https://github.com/rust-lang/compiler-team/issues/556, easier, this makes all of them use similar patterns and the same target base logic everywhere instead of picking bits from both. This also has some other smaller upsides like less stringly-typed functions.
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