Add `-Z self-profile` info to doc builds

[jyn514]

Currently, https://perf.rust-lang.org/detailed-query.html?commit=4ec27e4b79891b0ebc2ad71a3c4ac94f67d93f93&base_commit=09c9c9f7da72b774cc445c0f56fc0b9792a49647&benchmark=deep-vector-doc&run_name=full gives an error, even though https://perf.rust-lang.org/detailed-query.html?commit=4ec27e4b79891b0ebc2ad71a3c4ac94f67d93f93&base_commit=09c9c9f7da72b774cc445c0f56fc0b9792a49647&benchmark=deep-vector-check&run_name=full works fine. It would be great to track this info for rustdoc too.