RFR: Fix barrier set cast for aarch64

[William Kemper]

Simple fix for aarch64 (`ShenandoahBarrierSet` no longer extends from `CardTableBarrierSet`).

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Commit messages:
 - Fix barrier set cast for aarch64

Changes: https://git.openjdk.java.net/shenandoah/pull/33/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=33&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/33.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/33/head:pull/33

PR: https://git.openjdk.java.net/shenandoah/pull/33