RFR: 8252584: HotSpot Style Guide should permit alignas

[Julian Waters]

Add alignas to the permitted features set with some restrictions. (Thanks @kimbarrett for the help)

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Commit messages:
 - HotSpot Style Guide changes

Changes: https://git.openjdk.org/jdk/pull/11404/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11404&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8252584
  Stats: 94 lines in 2 files changed: 92 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11404.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11404/head:pull/11404

PR: https://git.openjdk.org/jdk/pull/11404