Auto merge of #143048 - Kobzol:bootstrap-check-stage-1, r=jieyouxu

Enforce in bootstrap that check must have stage at least 1

This PR is another step towards https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Proposal.20to.20cleanup.20stages.20and.20steps.20after.20the.20redesign/with/523586917, this time dealing with `x check`.

It enforces the invariant that:
- We check std stage N with rustc stage N
- We check everything else stage N with rustc stage N - 1

It creates a single function that prepares a proper build compiler for checking something, and also adds snapshot tests for various common check steps. Some obsolete code was also removed.

The default check stage also becomes 1, for all profiles. I tested manually that `x check std` with `download-ci-rustc` still works and doesn't build rustc locally.

Closes: rust-lang/rust#139170

r? `@ghost`

Author: bors