RFR: Bring jdk20 branch in sync with panama branch

[Maurizio Cimadamore]

This PR brings fixes from `panama` branch back to the `jdk20` branch.
We are doing this as part of getting ready to start shipping binary snapshot of the jdk20 version of jextract.

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Commit messages:
 - Backport of (7903314: Jextract doesn't honor pack pragmas)
 - Fix issue where zero-length padding is added
 - Fix jextract after removal of support for sub-byte layouts
 - Backport of (7903437: Improve translation strategy for function pointers)

Changes: https://git.openjdk.org/jextract/pull/114/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=114&range=00
  Stats: 336 lines in 16 files changed: 252 ins; 36 del; 48 mod
  Patch: https://git.openjdk.org/jextract/pull/114.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/114/head:pull/114

PR: https://git.openjdk.org/jextract/pull/114