RFR: 8287249: Zero: Missing BarrierSetNMethod::arm() method

[Zhengyu Gu]

Please review this trivial patch that adds missing method in Zero

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Commit messages:
 - 8287249: Zero: Missing BarrierSetNMethod::arm() method

Changes: https://git.openjdk.java.net/jdk/pull/8873/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8873&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287249
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8873.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8873/head:pull/8873

PR: https://git.openjdk.java.net/jdk/pull/8873