RFR: 8248132: ZGC: Unify handling of all OopStorage instances in root processing

[Stefan Karlsson]

Hi all,

Please review this patch to unify handling of all OopStorage instances 
in root processing.

https://cr.openjdk.java.net/~stefank/8248132/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8248132

This removes the explicit enumeration of "strong" OopStorages in ZGC. 
This is a step towards allowing the Runtime code to add new OopStorages 
without having to update all GCs.

Tested with tier1-3

Thanks,
StefanK