RFR: 8227226: Segmented array clearing for ZGC

[Sciampacone, Ryan]

http://cr.openjdk.java.net/~phh/8227226/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8227226

This patch introduces safe point checks into array clearing during allocation for ZGC.  The patch isolates the changes to ZGC as (in particular with the more modern collectors) the approach to incrementalizing or respecting safe point checks is going to be different.

The approach is to keep the region holding the array in the allocating state (pin logic) while updating the color to the array after checks.

Can I get a review?  Thanks.

Ryan