RFR: 8253689: [s390] Use flag kind diagnostic for platform specific flags

[Martin Doerr]

Current platform implementation (globals_s390.hpp) uses regular product flags for everything.
These 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.

CSR: https://bugs.openjdk.java.net/browse/JDK-8253691

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Commit messages:
 - 8253689: [s390] Use flag kind diagnostic for platform specific flags

Changes: https://git.openjdk.java.net/jdk/pull/414/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=414&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253689
  Stats: 18 lines in 1 file changed: 0 ins; 0 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/414.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/414/head:pull/414

PR: https://git.openjdk.java.net/jdk/pull/414