Auto merge of #124762 - madsmtm:refactor-apple-target-abi, r=lcnr,BlackHoleFox

Refactor Apple `target_abi`

This was bundled together with `Arch`, which complicated a few code paths and meant we had to do more string matching than necessary.

CC `@BlackHoleFox` as you've worked on the Apple target spec before

Related: Is there a reason why `Target`/`TargetOptions` use `StaticCow` for so many things, instead of an enum with defined values (and perhaps a catch-all case for custom target json files)? Tagging `@Nilstrieb,` as you might know?

Author: bors