RFR: 8209126: ZGC: ZMarkStackAllocator::is_initialized() never called

[Per Liden]

Failure to reserve the address space for the mark stacks doesn't 
terminate the VM during initialization. The reason is that 
ZHeap::is_initialized() doesn't call 
ZMarkStackAllocator::is_initialized(), which it should.

Bug: https://bugs.openjdk.java.net/browse/JDK-8209126
Webrev: http://cr.openjdk.java.net/~pliden/8209126/webrev.0

/Per