RFR: 8254719: ZGC: Clean up includes

Tue Oct 13 22:36:24 UTC 2020

I noticed quite a few includes in ZGC that are no longer needed and can be cleaned out. As a side-effect I noticed that
`stackWatermark.hpp` and `threadSMR.hpp` weren't self contained, so I had to fix that too.

Builds on all Oracle platforms.

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Commit messages:
 - 8254719: ZGC: Clean up includes

Changes: https://git.openjdk.java.net/jdk/pull/644/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=644&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254719
  Stats: 105 lines in 37 files changed: 17 ins; 71 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/644.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/644/head:pull/644

PR: https://git.openjdk.java.net/jdk/pull/644