runtime: add more fields to runtime.MemStats

[dgryski]

Some of these can help provide visibility into gc issues, since we don't otherwise have useful ways to see what's going on.

[dgryski]

More fields being added by #3424

[dgryski]

Closing until we have more things breaking because we don't have all the fields. I'm not keeping this bug open until we have 100% compatibility with Go. It's too vague for that. And I merged #4506 anyway.