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

[Stefan Karlsson]

On Wed, 14 Oct 2020 13:03:29 GMT, Per Liden <pliden at openjdk.org> wrote:

>> 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

Marked as reviewed by stefank (Reviewer).

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