RFR: JDK-8234502 : Merge GenCollectedHeap and SerialHeap

[Lei Zaakjyu]

JDK-8234502 : Merge GenCollectedHeap and SerialHeap

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Commit messages:
 - fix trialing whitespace
 - add copyright announsement
 - merge 'GenCollectedHeap' and 'SerialHeap'

Changes: https://git.openjdk.org/jdk/pull/16623/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16623&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8234502
  Stats: 2899 lines in 16 files changed: 1461 ins; 1418 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/16623.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16623/head:pull/16623

PR: https://git.openjdk.org/jdk/pull/16623