Don't compile rustc to self-test compiletest This was changed from stage 0 to 1 in https://github.com/rust-lang/rust/pull/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`