RFR: 8359373: Split stubgen initial blob into pre and post-universe blobs

[Andrew Dinn]

This PR adds a new preuniverse blob to the stubgen blobs set and relocates the initial fence stub to that blob.

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Commit messages:
 - 8359373: Split stubgen initial blob into pre and post-universe blobs

Changes: https://git.openjdk.org/jdk/pull/25784/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25784&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359373
  Stats: 147 lines in 17 files changed: 136 ins; 5 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/25784.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25784/head:pull/25784

PR: https://git.openjdk.org/jdk/pull/25784