Don't build the library and standard library before documenting them

Rustdoc doesn't require the build artifacts to generate the docs, and
especially in the case of rustc, it greatly increases the time needed to
run the build.

- Statically ensure that only the top_stage of a tool is documented

If another part of rustbuild tried to document a different stage, it
would run into errors because `check::Rustc` unconditionally uses the
top stage.

- Try building rustc instead of checking to avoid duplicate artifacts

Tries to workaround the following error:
```
error[E0464]: multiple matching crates for `rustc_ast`
  --> src/librustdoc/lib.rs:40:1
   |
40 | extern crate rustc_ast;
   | ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: candidates:
           crate `rustc_ast`: /checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_ast-6d7c193782263d89.rlib
           crate `rustc_ast`: /checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_ast-e5d09eda5beb759c.rmeta
```

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