RFR: 8199925: Break out GC selection logic from GCArguments to GCSelector

[Per Liden]

In an attempt to make WhiteBox more GC agnostic, and in turn make it 
easier to plugin new GC without touching a lot of non-GC code, this 
patch breaks out the GC selection logic from GCArguments to GCSelector. 
There are two reasons why I think this makes sense:
1) The GC selection logic is self-contained and fairly unrelated to the 
rest of the flags processing done by GCArguments and its GC-specific 
children.
2) Parts of the GC selection logic is needed by WhiteBox to answer 
questions about which GC are supported, selected, etc.

A follow up patch (JDK-8199927) will change WhiteBox (WB_CurrentGC, 
WB_AllSupportedGC, WB_GCSelectedByErgo), to use the GCSelector.

Bug: https://bugs.openjdk.java.net/browse/JDK-8199925
Webrev: http://cr.openjdk.java.net/~pliden/8199925/webrev.0

/Per