RFR: Final update refs roots are acting like Full GC

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/codecache-concevac-timing/webrev.02/

There is a regression introduced today. There is a call:

concurrentMark()->update_roots(ShenandoahCollectorPolicy::final_update_refs_roots);

...but the definition is:

 void update_roots(bool full_gc);

...which means it coerced SCP::final_update_refs_roots to "true"! This makes a
code cache update during final UR, wasting time.

Thanks,
-Aleksey