RFR: 8232650: ZGC: Add initialization hooks for OS specific code

[Stefan Karlsson]

Hi all,

Please review this patch to add initialization hooks for OS specific code.

https://cr.openjdk.java.net/~stefank/8232650/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8232650

These hooks are needed to for a Windows port. ZInitialize allows 
syscalls to be dynamically resolved. ZVirtualMemory allows callbacks 
from 8232649 to be initialized.

Thanks,
StefanK