RFR: 8359373: Split stubgen initial blob into pre and post-universe blobs [v2]

[Andrew Dinn]

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

Andrew Dinn has updated the pull request incrementally with one additional commit since the last revision:

  locate x86 stub generation methods in class

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25784/files
  - new: https://git.openjdk.org/jdk/pull/25784/files/d7b59889..8593475b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25784&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25784&range=00-01

  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 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