Auto merge of #124491 - madsmtm:target_vendor-apple, r=workingjubilee

Use `target_vendor = "apple"` instead of `target_os = "..."`

Use `target_vendor = "apple"` instead of `all(target_os = "macos", target_os = "ios", target_os = "tvos", target_os = "watchos", target_os = "visionos")`.

The apple targets are quite close to being identical, with iOS, tvOS, watchOS and visionOS being even closer, so using `target_vendor` when possible makes it clearer when something is actually OS-specific, or just Apple-specific.
Note that `target_vendor` will [be deprecated in the future](rust-lang/rust#100343), but not before an alternative (like `target_family = "apple"`) is available.

While doing this, I found various inconsistencies and small mistakes in the standard library, see the commits for details. Will follow-up with an extra PR for a similar issue that need a bit more discussion. EDIT: rust-lang/rust#124494

Since you've talked about using `target_vendor = "apple"` in the past:
r? workingjubilee

CC `@simlay,` `@thomcc`
`@rustbot` label O-macos O-ios O-tvos O-watchos O-visionos