RFR: 8325049: stubGenerator_ppc.cpp should use alignas

[Julian Waters]

Please review a trivial change to make stubGenerator_ppc.cpp use the well defined alignas instead of the aligned attribute. This was the only site I could find that does not use ATTRIBUTE_ALIGNED and instead independently defines the aligned attribute on its own, so I swapped it to use alignas instead

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Commit messages:
 - 8325049

Changes: https://git.openjdk.org/jdk/pull/17652/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17652&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325049
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17652.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17652/head:pull/17652

PR: https://git.openjdk.org/jdk/pull/17652