RFR: Bring jdk20 branch in sync with panama branch [v2]

[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.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Update README to reflect new layout

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Changes:
  - all: https://git.openjdk.org/jextract/pull/114/files
  - new: https://git.openjdk.org/jextract/pull/114/files/690a70df..9b4f33e3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=114&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=114&range=00-01

  Stats: 123 lines in 1 file changed: 58 ins; 50 del; 15 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