[jdk25u] RFR: 8369979: Flag UsePopCountInstruction was accidentally disabled on PPC64

[David Briemann]

This pull request contains a backport of commit [9b9559a2](https://github.com/openjdk/jdk/commit/9b9559a2e33827126e1aeab7bf6f4861acaae109) from the [openjdk/jdk](https://git.openjdk.org/jdk) repository.

The commit being backported was authored by David Briemann on 17 Oct 2025 and was reviewed by Andrew Haley and Martin Doerr.

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Commit messages:
 - Backport 9b9559a2e33827126e1aeab7bf6f4861acaae109

Changes: https://git.openjdk.org/jdk25u/pull/326/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=326&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369979
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk25u/pull/326.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/326/head:pull/326

PR: https://git.openjdk.org/jdk25u/pull/326