RFR: Fix 32-bit builds

[Aleksey Shipilev]

Currently fails with:

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Commit messages:
 - Fix 32-bit builds

Changes: https://git.openjdk.java.net/shenandoah/pull/49/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=49&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/49.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/49/head:pull/49

PR: https://git.openjdk.java.net/shenandoah/pull/49