Use materialize_sysroot in rustdoc

Author: bjorn3

Diff for: src/librustdoc/config.rs

@@ -732,10 +732,7 @@ impl Options {
         let target = parse_target_triple(early_dcx, matches);
         let maybe_sysroot = matches.opt_str("sysroot").map(PathBuf::from);
 
-        let sysroot = match &maybe_sysroot {
-            Some(s) => s.clone(),
-            None => rustc_session::filesearch::get_or_default_sysroot(),
-        };
+        let sysroot = rustc_session::filesearch::materialize_sysroot(maybe_sysroot.clone());
 
         let libs = matches
             .opt_strs("L")