[foreign-jextract] RFR: 8245066: struct/union/array address accessor should be x$ADDR() for consistency

[Athijegannathan Sundararajan]

* renamed struct/union/array accessor as x$ADDR
* Piggybacking to add a jextract based test for bug 8244938

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Commit messages:
 - 8245066: struct/union/array address accessor should be x$ADDR() for consistency

Changes: https://git.openjdk.java.net/panama-foreign/pull/166/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/166/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8245066
  Stats: 150 lines in 7 files changed: 124 ins; 14 del; 12 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/166.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/166/head:pull/166

PR: https://git.openjdk.java.net/panama-foreign/pull/166