RFR: Fix barrier set cast for aarch64 [v2]

[William Kemper]

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

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Simplify barrier casting

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/33/files
  - new: https://git.openjdk.java.net/shenandoah/pull/33/files/062c68b1..941c3ae9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=33&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=33&range=00-01

  Stats: 6 lines in 2 files changed: 0 ins; 2 del; 4 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