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.

Author: cuviper

compiler-builtins/Cargo.toml

@@ -19,6 +19,8 @@ links = "compiler-rt"
 bench = false
 doctest = false
 test = false
+# make sure this crate isn't included in public standard library docs
+doc = false
 
 [dependencies]
 core = { path = "../../core", optional = true }