RFR: 8255760: Shenandoah: match constants style in ShenandoahMarkTask fallback

[Roman Kennke]

On Mon, 2 Nov 2020 13:55:59 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> JDK-8255457 cleaned up the 64-bit version of ShenandoahMarkTask. However, there is also the 32-bit fallback without any tricky mechanics. Still, it should match the style of the optimized version by using `static const`-s instead of enums. I missed that during the JDK-8255457 work.
> 
> Additional testing:
>  - [x] Linux x86_32 `hotspot_gc_shenandoah`

Looks good to me!

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Marked as reviewed by rkennke (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1006