RFR: JDK-8314777: GenShen: Alias young and old marking bits to legacy Shenandoah marking bit in gc state

[Kelvin Nilsen]

This change sets the MARKING bit of _gc_state whenever either Young marking or Old marking is active.  With this change, barrier code more closely resembles the code in the original single-generation Shenandoah.  The performance impact is negligible.  The primary benefit is to simplify code reviews and clarify that the addition of generational mode to Shenandoah does not negatively impact performance of single-generation Shenandoah.

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Commit messages:
 - Fix whitespace
 - More reviewer requests
 - Fix white space error
 - Fiddle with format to fix jcheck
 - Respond to reviewer feedback
 - Fixup bit-manipulation errors for multi-bit gc state
 - Make MARKING bit an alias for YOUNG_MARKING | OLD_MARKING

Changes: https://git.openjdk.org/shenandoah/pull/309/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=309&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314777
  Stats: 65 lines in 11 files changed: 27 ins; 8 del; 30 mod
  Patch: https://git.openjdk.org/shenandoah/pull/309.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/309/head:pull/309

PR: https://git.openjdk.org/shenandoah/pull/309