Auto merge of #112256 - jyn514:faster-mingw-check, r=Mark-Simulacrum

Don't compile rustc to self-test compiletest

This was changed from stage 0 to 1 in rust-lang/rust#108905, but I'm not sure why. Change it to `top_stage` instead to allow people to choose the stage.

This should save quite a bit of time in the `mingw-check` builder, which explicitly runs `x test --stage 0 compiletest`.

Note that this also fixes a latent bug that depended on running `x build compiler` before `x doc compiler`, as well as a couple cleanups related to symlinks (which made the latent bug easier to find).

cc `@pietroalbini`

Author: bors