[jdk20] RFR: 8298225: [AIX] Disable PPC64LE continuations on AIX

[Tyler Steele]

This small change adds an import to ppc.ad to allow it to find Contiuations::enabled, and sets VMContinuations to false on AIX.

Thanks to @TheRealMDoerr for suggestions to a previous version of this PR.

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Commit messages:
 - Modify ppc.ad
 - Set VMContinuations to false on AIX

Changes: https://git.openjdk.org/jdk20/pull/4/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20&pr=4&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298225
  Stats: 5 lines in 2 files changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk20/pull/4.diff
  Fetch: git fetch https://git.openjdk.org/jdk20 pull/4/head:pull/4

PR: https://git.openjdk.org/jdk20/pull/4