RFR: JDK-8296743: Tighten Class.getModifiers spec for array classes

[Joe Darcy]

Update the spec of Class.getModifiers to match long-standing behavior for primitive and array classes. Remove unneeded implementation flexibility with regard to setting other bit positions. This work was prompted to better support anticipated future Valhalla changes.

Please also review the CSR: https://bugs.openjdk.org/browse/JDK-8297237

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Commit messages:
 - JDK-8296743: Tighten Class.getModifiers spec for array classes

Changes: https://git.openjdk.org/jdk/pull/11229/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11229&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296743
  Stats: 152 lines in 2 files changed: 141 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/11229.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11229/head:pull/11229

PR: https://git.openjdk.org/jdk/pull/11229