RFR: 8209894: ZGC: Cap number of GC workers based on heap size

[Per Liden]

The ergonomics for selecting number of GC workers does not take 
HeapSizePerGCThread into account. This is sub-optimal and can even lead 
to pre-mature OOME when using a tiny heap on a very large machine, since 
ZGC reserves 2M of the heap per GC worker.

Bug: https://bugs.openjdk.java.net/browse/JDK-8209894
Webrev: http://cr.openjdk.java.net/~pliden/8209894/webrev.0

/Per