RFR: 8199927: Make WhiteBox more GC agnostic

[Per Liden]

Make WhiteBox more GC agnostic by having it use the GCSelector to answer 
questions about which GCs are supported, selected, etc.

Going forward, this will make it easier to plug in new GC without having 
to touch and adjust the WhiteBox code.

Bug: https://bugs.openjdk.java.net/browse/JDK-8199927
Webrev: http://cr.openjdk.java.net/~pliden/8199927/webrev.0

/Per