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

[Vladimir Kozlov]

On Thu, 12 Jun 2025 17:35:43 GMT, Andrew Dinn <adinn at openjdk.org> wrote:

>> 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

Thank you @adinn for this change. I have only one comment.

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PR Review: https://git.openjdk.org/jdk/pull/25784#pullrequestreview-2922124839