Fix handling of no_std targets in doc::Std step

[Kobzol]

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.

r? @jieyouxu

[jieyouxu]

I think some tests need reblessing tho.

[Kobzol]

Yeah, I'll actually need to reimplement the tests, the way I did host normalization was not working outside x64.