RFR: 8321063: AArch64: Zero build fails after JDK-8320368

[Aleksey Shipilev]

Simple: we need a definition for Zero, because we cannot reach the AArch64-specific one in `src/cpu/aarch64`.

Additional testing:
 - [x] MacOS AArch64 Zero build now passes
 - [ ] Extensive build matrix of Zero and Server builds

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/16897/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16897&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321063
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16897.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16897/head:pull/16897

PR: https://git.openjdk.org/jdk/pull/16897