RFR: 8253971: ZGC: Flush mark stacks after processing concurrent roots

[Per Liden]

I looked into why almost all mark cycles see non-zero "mark completions". In other words, we almost always have some
amount of mark work left to handle in the mark end pause. It turns out that worker threads don't flush their mark
stacks in ZMarkConcurrentRootsTask::work(), which means they can hide work (in their thread local mark stacks) until
those stacks are finally flushed out in ZMark::try_end(). The reason work can be hidden is that the set of worker
threads executing ZMarkConcurrentRootsTask is not necessarily the same set of worker threads executing ZMarkTask. As a
result, the mark end pause often becomes longer than it otherwise would have.

After fixing this, I did some tests with Dacapo, which shows the following improvement:

Before: Mark End Pause (avg/max): 0.391 / 1.142 ms
After: Mark End Pause (avg/max): 0.130 / 0.294 ms

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Commit messages:
 - 8253971: ZGC: Flush mark stacks after processing concurrent roots

Changes: https://git.openjdk.java.net/jdk/pull/495/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=495&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253971
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/495.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/495/head:pull/495

PR: https://git.openjdk.java.net/jdk/pull/495