RFR: 7903239: ofAddress factory of function pointer type is wrong for struct returns

[Jorn Vernee]

This is a more up-to-date version of: https://github.com/openjdk/jextract/pull/58 which takes the recent re-writes into account.

For callbacks that return a struct by-value, we need a SegmentAllocator. We can re-use the Arena we use as a scope for the function pointer we are wrapping. If a user wants a different allocator in different circumstances, they can just re-wrap the pointer and pass another Arena.

-------------

Commit messages:
 - pass allocator to callbacks that needs it

Changes: https://git.openjdk.org/jextract/pull/162/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=162&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903239
  Stats: 138 lines in 4 files changed: 137 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/162.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/162/head:pull/162

PR: https://git.openjdk.org/jextract/pull/162