[lilliput-jdk21u:lilliput] RFR: 8316687: [Lilliput/JDK21] Various cleanups

[Roman Kennke]

Review of upstream diff has revealed various minor issues. Let's fix them collectively.

Testing:
 - [x] hotspot:tier1 +UCOH

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Commit messages:
 - Various cleanups
 - Cleanup and add asserts
 - Move gap init into allocate_header() (x86); Remove unnecessary code (aarch64)

Changes: https://git.openjdk.org/lilliput-jdk21u/pull/9/files
 Webrev: https://webrevs.openjdk.org/?repo=lilliput-jdk21u&pr=9&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316687
  Stats: 63 lines in 14 files changed: 22 ins; 29 del; 12 mod
  Patch: https://git.openjdk.org/lilliput-jdk21u/pull/9.diff
  Fetch: git fetch https://git.openjdk.org/lilliput-jdk21u.git pull/9/head:pull/9

PR: https://git.openjdk.org/lilliput-jdk21u/pull/9