RFR: 8209376: ZGC: Move ZMarkStackAllocator into a separate file

[Kim Barrett]

> On Aug 13, 2018, at 3:53 AM, Per Liden <per.liden at oracle.com> wrote:
> 
> Break out the mark stack allocation logic from ZMarkStack.[ch]pp into ZMarkStackAllocator.[ch].pp.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8209376
> Webrev: http://cr.openjdk.java.net/~pliden/8209376/webrev.0
> 
> /Per

Looks good.