RFR: 8343658: Parallel: Implement block_start for Young generation

[Albert Mingkun Yang]

Simple block_start implementation for Parallel young-gen. Related to https://github.com/openjdk/jdk/pull/21870

Test: tier1-3

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Commit messages:
 - pgc-block-start

Changes: https://git.openjdk.org/jdk/pull/21919/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21919&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343658
  Stats: 28 lines in 3 files changed: 24 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21919.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21919/head:pull/21919

PR: https://git.openjdk.org/jdk/pull/21919