RFR: 8209375: ZGC: Use dynamic base address for mark stack space

[Kim Barrett]

> On Aug 13, 2018, at 3:53 AM, Per Liden <per.liden at oracle.com> wrote:
> 
> We're currently using a fixed address space range for the mark stack space, both in terms of it's base address and it's size. This isn't really needed, we could just as well let os::reserve_memory() pick and address range for us and only reserve ZMarkStackSpaceLimit bytes, instead of 1TB.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8209375
> Webrev: http://cr.openjdk.java.net/~pliden/8209375/webrev.0
> 
> /Per

Looks good.