RFR: 8278598: AlignmentReserve is repeatedly reinitialized

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that makes initialization of `AlignmentReserve` a one-time thing; after looking at the suggested change and the code I opted to use a static in `CollectedHeap` that can be queried by its users (`PLAB` and `ThreadLocalAllocBuffer`). This seemed nicer to me.

Testing: gha, running gcbasher with all collectors (notice that there is an assert in the getter that checks for initialization before retrieval, so this should be sufficient)

Thanks,
  Thomas

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Commit messages:
 - initial version

Changes: https://git.openjdk.java.net/jdk/pull/8808/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8808&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278598
  Stats: 37 lines in 5 files changed: 15 ins; 14 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8808.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8808/head:pull/8808

PR: https://git.openjdk.java.net/jdk/pull/8808