RFR: 8254719: ZGC: Clean up includes [v3]

[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 2

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/644/files
  - new: https://git.openjdk.java.net/jdk/pull/644/files/dcbde4f1..3fd681f3

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=644&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=644&range=01-02

  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 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