RFR: CODETOOLS-7902869: JOL: Support JDK 15+ field layouter simulation

[Aleksey Shipilev]

JDK-8237767 added a new layouter. JOL needs to simulate it as well.

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Commit messages:
 - Minor cleanup
 - Merge branch 'master' into jdk15-layouter
 - Add JDK 15 Hotspot layouter simulation

Changes: https://git.openjdk.java.net/jol/pull/14/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jol&pr=14&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902869
  Stats: 169 lines in 9 files changed: 130 ins; 15 del; 24 mod
  Patch: https://git.openjdk.java.net/jol/pull/14.diff
  Fetch: git fetch https://git.openjdk.java.net/jol pull/14/head:pull/14

PR: https://git.openjdk.java.net/jol/pull/14