RFR: Fix barrier set cast for aarch64 [v2]

[Roman Kennke]

On Fri, 30 Apr 2021 00:34:36 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> 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

Please try /integrate :-)

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