RFR: 8327365: Serial: Move _gc_stats to TenuredGeneration [v2]

[Albert Mingkun Yang]

> Simple refactoring to move a field from superclass to subclass.

Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - review
 - Merge branch 'master' into s1-flatten-gc-stat
 - s1-flatten-gc-stat

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18122/files
  - new: https://git.openjdk.org/jdk/pull/18122/files/3d7c1237..8276cd59

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18122&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18122&range=00-01

  Stats: 328 lines in 18 files changed: 239 ins; 61 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/18122.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18122/head:pull/18122

PR: https://git.openjdk.org/jdk/pull/18122