[jdk25u-dev] RFR: 8307495: Specialize atomic bitset functions for aix-ppc

[David Briemann]

This pull request contains a backport of commit [c9cbd31f](https://github.com/openjdk/jdk/commit/c9cbd31f8575a25c4decd68dc645378c5ba2bad0) from the [openjdk/jdk](https://git.openjdk.org/jdk) repository.

The commit being backported was authored by David Briemann on 15 Oct 2025 and was reviewed by Martin Doerr and Richard Reingruber.

A merge conflict was resolved, that was created by renaming files: https://github.com/openjdk/jdk/pull/27245
Otherwise the backport is clean.

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Commit messages:
 - Backport c9cbd31f8575a25c4decd68dc645378c5ba2bad0

Changes: https://git.openjdk.org/jdk25u-dev/pull/24/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u-dev&pr=24&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307495
  Stats: 1548 lines in 6 files changed: 649 ins; 878 del; 21 mod
  Patch: https://git.openjdk.org/jdk25u-dev/pull/24.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u-dev.git pull/24/head:pull/24

PR: https://git.openjdk.org/jdk25u-dev/pull/24