RFR: 8255036: Shenandoah: need to reset GC state for root verifier

[Zhengyu Gu]

Shenandoah has ShenandoahGCStateResetter to reset gc state for verifier to avoid triggering barriers inadvertently, root verifier should do the same.

Also, SH::_concurrent_weak_root_in_progress flag now has implication for native barrier, ShenandoahGCStateResetter should reset this flag too.

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Commit messages:
 - Merge branch 'master' into JDK-8255036-gcstate_resetter
 - 8255036: Shenandoah: need to reset GC state for root verifier

Changes: https://git.openjdk.java.net/jdk/pull/781/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=781&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255036
  Stats: 43 lines in 3 files changed: 26 ins; 17 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/781.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/781/head:pull/781

PR: https://git.openjdk.java.net/jdk/pull/781