RFR (S): 8219369: Add named constants for iterating ExtRootScan phases

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that has been suggested as part of
another review to improve readability of the ExtRootScan sub-phase
printing?

In particular, it introduces a few new named constants in the
GCParPhases enum that indicate start and end of the ext root scan sub-
phases. These are then used in the iteration.

CR:
https://bugs.openjdk.java.net/browse/JDK-8219369
Webrev:
http://cr.openjdk.java.net/~tschatzl/8219369/webrev/
testing:
hs-tier1,hs-tier2 (includes log output testing for completeness)

Thanks,
  Thomas