Break out GC selection logic from GCArguments to GCConfig

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 GCConfig. 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 will change WhiteBox (WB_CurrentGC, WB_AllSupportedGC, WB_GCSelectedByErgo), to use the GCConfig.