RFR(xs): 8198271: Make CollectedHeap::create_heap_space_summary() virtual

[Per Liden]

This patch makes CollectedHeap::create_heap_space_summary() virtual, to 
allow a GC to override it. The current default implementation assumes 
that the "heap space" is the same as the "reserved_region", which isn't 
necessarily true. ZGC (and possibly Shenandoah in the future?) have a 
different heap layout and thus needs to override this function.

Bug: https://bugs.openjdk.java.net/browse/JDK-8198271
Webrev: http://cr.openjdk.java.net/~pliden/8198271/webrev.0/

/Per