RFR: 8329494: Serial: Merge GenMarkSweep into MarkSweep

[Guoxiong Li]

Hi all,

This patch merges the class `GenMarkSweep` into `MarkSweep`. Just simply move. It may be better to merge the class `DeadSpacer` and `Compacter` into `MarkSweep` and then rename `MarkSweep`. The `MarkSweep` will be renamed at [JDK-8329521](https://bugs.openjdk.org/browse/JDK-8329521). And the `DeadSpacer` and `Compacter` may be refactored at [JDK-8305896](https://bugs.openjdk.org/browse/JDK-8305896) and [JDK-8305898](https://bugs.openjdk.org/browse/JDK-8305898), so I don't refactor them now.

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-8329494

Changes: https://git.openjdk.org/jdk/pull/18589/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18589&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329494
  Stats: 1112 lines in 8 files changed: 513 ins; 589 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/18589.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18589/head:pull/18589

PR: https://git.openjdk.org/jdk/pull/18589