RFR: 8259252: Shenandoah: Shenandoah build failed on AArch64 after JDK-8258459

[Zhengyu Gu]

Please review this trivial change that fixes build failure after JDK-8258459

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Commit messages:
 - Fix build failure

Changes: https://git.openjdk.java.net/jdk/pull/1946/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1946&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259252
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1946.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1946/head:pull/1946

PR: https://git.openjdk.java.net/jdk/pull/1946