RFR: 8327365: Serial: Move _gc_stats to TenuredGeneration [v3]

[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 five additional commits since the last revision:

 - review
 - Merge branch 'master' into s1-flatten-gc-stat
 - 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/8276cd59..95c08cdc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18122&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18122&range=01-02

  Stats: 69123 lines in 616 files changed: 1089 ins; 66973 del; 1061 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