RFR: 8291912: Introduce per-allocation target struct for members in G1PLABAllocator

[Thomas Schatzl]

Hi all,

  can I get reviews for this change that refactors the many per-destination arrays in `PLABAllocator` into an array of struct as I promised in the review for [JDK-8288966](https://bugs.openjdk.org/browse/JDK-8288966)/[PR#9726](https://github.com/openjdk/jdk/pull/9726)? I think that makes the code easier to follow since there is less clutter.

Testing: tier1-5, no performance differences

Thanks,
  Thomas

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Depends on: https://git.openjdk.org/jdk/pull/9726

Commit messages:
 - initial version

Changes: https://git.openjdk.org/jdk/pull/9818/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9818&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291912
  Stats: 115 lines in 3 files changed: 59 ins; 30 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/9818.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9818/head:pull/9818

PR: https://git.openjdk.org/jdk/pull/9818