RFR: 8329529: Serial: Move _saved_mark_word out of ContiguousSpace

[Guoxiong Li]

Hi all,

This patch removes the field `ContiguousSpace::_saved_mark_word` and its related methods, adds the corresponding fields to `DefNewGeneration` and `TenuredGeneration`, and moves the method `ContiguousSpace::oop_since_save_marks_iterate` to `Generation`. Many usages are adjusted as well.

An alternative way is add a class which extends the `ContiguousSpace` to place `_saved_mark_word` and its related methods. But this way would increase the class hierarchy. Not really know whether it deserves to do that.

The tests `make test-tier1_gc` passed locally. Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8329529

Changes: https://git.openjdk.org/jdk/pull/18603/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18603&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329529
  Stats: 132 lines in 13 files changed: 56 ins; 52 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/18603.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18603/head:pull/18603

PR: https://git.openjdk.org/jdk/pull/18603