RFR: 8199927: Make WhiteBox more GC agnostic

[Per Liden]

Updated webrev to reflect that GCSelector changed name to GCConfig in 
the patch for JDK-8199925.

http://cr.openjdk.java.net/~pliden/8199927/webrev.1

/Per

On 03/21/2018 09:14 AM, Per Liden wrote:
> 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