RFR: 8346869: [AIX] Add regression test for handling 4 Byte aligned doubles in structures

[Martin Doerr]

[JDK-8317545](https://bugs.openjdk.org/browse/JDK-8317545) introduced code for supporting 4 Byte aligned doubles: https://github.com/openjdk/jdk/blob/60e0730a3ba26180d0eb2cd278e389c3e70fec5f/src/java.base/share/classes/jdk/internal/foreign/abi/ppc64/aix/AixPPC64Linker.java#L59
We should test this case. The new test is implemented platform agnostic, such that it can run on all platforms.

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Commit messages:
 - 8346869: [AIX] Add regression test for handling 4 Byte aligned doubles in structures

Changes: https://git.openjdk.org/jdk/pull/22889/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22889&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346869
  Stats: 178 lines in 2 files changed: 178 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22889.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22889/head:pull/22889

PR: https://git.openjdk.org/jdk/pull/22889