RFR: Fix Zero builds

[Aleksey Shipilev]

There are many build failures in Zero configurations, all look like this:

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Commit messages:
 - Fix Zero builds

Changes: https://git.openjdk.java.net/shenandoah/pull/58/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=58&range=00
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/58.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/58/head:pull/58

PR: https://git.openjdk.java.net/shenandoah/pull/58