RFR: Update code to use address target layouts

[Maurizio Cimadamore]

This patch tweaks jextract to replace usages of `OfAddress.asUnbounded` with:

`withTargetLayout(sequenceLayout(JAVA_BYTE))`

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Commit messages:
 - Update code to use target layouts

Changes: https://git.openjdk.org/jextract/pull/100/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=100&range=00
  Stats: 11 lines in 4 files changed: 7 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jextract/pull/100.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/100/head:pull/100

PR: https://git.openjdk.org/jextract/pull/100