RFR: 8221786: ZGC: Increase max heap size to 16TB

[Per Liden]

Increase max heap size to 16TB. We could just bump 
ZPlatformAddressOffsetBits to 44, but we should take the opportunity to 
make this slightly more dynamic and set ZPlatformAddressOffsetBits at 
runtime. As a start, I propose that we limit the dynamic behavior to an 
address space reservation of 4TB, 8TB or 16TB (per heap view), depending 
on what -Xmx is set to. In the future we could have a more elaborate 
scheme if we find a need for that.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221786
Webrev: http://cr.openjdk.java.net/~pliden/8221786/webrev.0

/Per