RFR: 8260736: Shenandoah: Cleanup includes in ShenandoahGC and families [v3]

[Zhengyu Gu]

> 8260736: Shenandoah: Cleanup includes in ShenandoahGC and families

Zhengyu Gu has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:

 - Merge master
 - Added back vmThread.hpp
 - update
 - Merge branch 'master' into JDK-8260736-cleanup-includes-gc
 - update
 - init

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Changes: https://git.openjdk.java.net/jdk/pull/2339/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2339&range=02
  Stats: 35 lines in 10 files changed: 2 ins; 28 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2339.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2339/head:pull/2339

PR: https://git.openjdk.java.net/jdk/pull/2339