We need a reliable, platform-independent way to find out the extent of the user-mappable address space. This is needed e.g. in the aslr-aware new class space reservation. At the moment, we assume either 128 TB or 2^48, but we may run on big and small machines. Aarch64, for instance, can have either 39, 48 or 52 bit address space.

ZGC already probes the extent of the address space. Maybe that technique could be re-used for the general hotspot.

Note that we already have an abstraction for the lower limit of the address space - os::vm_min_address(), a new implementation should have the same form.