Rollup merge of #143615 - Kobzol:doc-std, r=jieyouxu

Fix handling of no_std targets in `doc::Std` step

The previous logic was wrong for no_std targets, it just didn't do anything. The logic was added there because by default, the `Std` step would otherwise have a list of all std crates to check, but these would fail for no_std targets. What has to happen instead is to select the default set of packages to check/doc/build, which currently happens in the `std_cargo` function, but the `self.crates` list was overriding that.

In general, using `crates: Vec<String>` in the `Std` steps is quite fishy, because it's difficult to distinguish between all crates (either they are all enumerated or `crates` is empty) and the default (e.g. `x <kind> [library]`) vs a subset (e.g. `x <kind> core`). I wanted to improve that using an enum that would distinguish these situations, avoid passing `-p` for all of the crates explicitly, and unify the selection of packages to compile/check/... in `std_cargo`, based on this enum.

However, I found out from some other bootstrap comments that when you pass `-p` explicitly for all crates, cargo behaves differently (apparently for check it will also check targets/examples etc. with `-p`, but not without it). Furthermore, the doc step has a special case where it does not document the `sysroot` package. So as usually, unifying this logic would get into some edge cases... So instead I opted for a seemingly simpler solution, where I try to prefilter only two allowed crates (core and alloc) for no_std targets in the `std_crates_for_run_make` function.

It's not perfect, but I think it's better than the status quo (words to live by when working on bootstrap...).

Fixes [this Zulip topic](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/docs.20for.20non-host.20targets.3F).

r? `@jieyouxu`

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