ZGC: Increase max heap size to 16TB

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.