RFR: JDK-8328272: [AIX] Use flag kind "diagnostic" for platform specific flags

[Joachim Kern]

Current platform implementation (globals_aix.hpp) uses regular product flags for almost everything.
Most platform specific flags were never intended for official support. They are only there to diagnose issues and find workarounds.
So flag kind "diagnostic" fits better for them.

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Commit messages:
 - JDK-8328272

Changes: https://git.openjdk.org/jdk/pull/18337/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18337&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328272
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/18337.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18337/head:pull/18337

PR: https://git.openjdk.org/jdk/pull/18337