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

[Aleksey Shipilev]

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`

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Commit messages:
 - 8255760: Shenandoah: match constants style in ShenandoahMarkTask fallback

Changes: https://git.openjdk.java.net/jdk/pull/1006/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1006&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255760
  Stats: 7 lines in 1 file changed: 1 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1006.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1006/head:pull/1006

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