Shenandoah: Add missing Shenandoah implementation in WB_isObjectInOldGen

The test gc/TestReferenceRefersTo.java fails with Shenandoah because of a missing implementation in WB_isObjectInOldGen:

# Internal Error (/home/rkennke/src/openjdk/jdk/src/hotspot/share/gc/shared/collectedHeap.hpp:207), pid=511307, tid=511422
# assert(kind == heap->kind()) failed: Heap kind 6 should be 1

This is introduced by JDK-8188055.