Provide a switch to force the class space to a specific location

To test compressed Klass pointer encoding and other metaspace interna, it would be nice to be able to force location of the class space to a specific location. This would help with analysis of problems like JDK-8261552 or JDK-8265705 - errors in Klass* encoding/decoding which managed to stay unnoticed for an astonishingly long time.

For simplicity, it would be sufficient to only have this ability if CDS is disabled (Xshare=off), since CCS allocation is tied to CDS location otherwise and that would be more difficult to disentangle.