Integrated: sample fixes for Arena API change.

[Athijegannathan Sundararajan]

All samples run fine on MacOS (except for OpenGL which has been broken on Mac for a while now)

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

Commit messages:
 - sample fixes for Arena API change.

Changes: https://git.openjdk.org/jextract/pull/103/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=103&range=00
  Stats: 56 lines in 27 files changed: 3 ins; 0 del; 53 mod
  Patch: https://git.openjdk.org/jextract/pull/103.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/103/head:pull/103

PR: https://git.openjdk.org/jextract/pull/103