RFR: 8343218: Disable allocating interface and abstract classes in non-class metaspace

[Coleen Phillimore]

This change introduces a diagnostic flag to disable the change to allocate interface and abstract Klass metadata in the non-class metaspace.  Testing for JDK 25 has completed with this feature in place, but there may be cases where we should disable this.  A future change will be to remove this code.
Tested with tier1-4 with the flag off, and 1-4 with the flag on.

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Commit messages:
 - 8343218: Disable allocating interface and abstract classes in non-class metaspace

Changes: https://git.openjdk.org/jdk/pull/26579/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26579&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343218
  Stats: 7 lines in 3 files changed: 4 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26579.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26579/head:pull/26579

PR: https://git.openjdk.org/jdk/pull/26579