RFR: 8254719: ZGC: Clean up includes [v2]

[Per Liden]

> 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.

Per Liden has updated the pull request incrementally with one additional commit since the last revision:

  Review stefank

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/644/files
  - new: https://git.openjdk.java.net/jdk/pull/644/files/78b772a9..dcbde4f1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=644&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=644&range=00-01

  Stats: 19 lines in 13 files changed: 12 ins; 5 del; 2 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