RFR: 7903632: struct accessor info is not derived from toplevel layout decl [v2]

[Maurizio Cimadamore]

> This PR dials back the code generation updates a little when it comes to struct getter/setter, so that both their layouts and offsets is now derived from the toplevel struct layout - using layout paths, as before.
> 
> This allows clients to e.g. tweak layouts in a single place (the topelevel struct layout declaration) to adjust things such as endianness.

Maurizio Cimadamore has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge branch 'panama' into derived_field_offests_layouts
 - Initial push

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Changes: https://git.openjdk.org/jextract/pull/181/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=181&range=01
  Stats: 34 lines in 2 files changed: 28 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jextract/pull/181.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/181/head:pull/181

PR: https://git.openjdk.org/jextract/pull/181