RFR: 7903878: jextract should derive struct field offsets from struct layouts

[Nizar Benalla]

Please read the JBS issue for more details.

Currently jextract emits hardwired offsets, it would be better if the offsets could be computed dynamically from the struct layouts.

Passes all tests, unsure if this patch requires new tests.
Note: I used `fieldElementPaths(field.name())))` instead of the `javaName` to deal with nested structs.


It seems like GitHub actions are broken for Ubuntu? will look into this next week if necessary.

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Commit messages:
 - jextract should derive struct field offsets from struct layouts

Changes: https://git.openjdk.org/jextract/pull/262/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=262&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903878
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jextract/pull/262.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/262/head:pull/262

PR: https://git.openjdk.org/jextract/pull/262