RFR: 8024927: Nashorn performance regression with CompressedOops

[Coleen Phillimore]

Summary: Allocate compressed class space at end of Java heap.  For small 
heap sizes, without CDS, reserve some space under 32G so compressed 
classes can have the same (more favorable) compression algorithm as oops.

This change may have to be reverted when we implement extending 
compressed class spaces in the future, but it gets back the performance 
of these nashorn benchmarks, and seems to make sense for small heaps.

open webrev at http://cr.openjdk.java.net/~coleenp/8024927/
bug link https://bugs.openjdk.java.net/browse/JDK-8024927

Tested with ute vm.quick.testlist vm.mlvm.testlist on linux/x64 
solaris/x64 and windows/x64 (in progress).  Also running refworkload.

Thanks,
Coleen