RFR 8166848: Performance bug: SystemDictionary - optimization

[coleen.phillimore at oracle.com]

> Summary: Check instead that a bucket isn't 10x the average
>
> See bug and linked bugs for more details.
>
> Tested with RBT nightly tests (tier2-5).
>
> open webrev at http://cr.openjdk.java.net/~coleenp/8166848.01/webrev
> bug link https://bugs.openjdk.java.net/browse/JDK-8166848
>
> Thanks,
> Coleen