Disable docs for `compiler-builtins` and `sysroot`
Bootstrap already had a manual doc filter for the `sysroot` crate, but
other library crates keep themselves out of the public docs by setting
`[lib] doc = false` in their manifest. This seems like a better solution
to hide `compiler-builtins` docs, and removes the `sysroot` hack too.
Fixesrust-lang/rust#143215 (after backport)
```@rustbot``` label beta-nominated
6c4502d97d