RFR(sh/jdk11): Shenandoah: Move markBitMap.* and parallelCleaning.* to shenandoah subdirectory

[Roman Kennke]

This moves markBitMap.* and parallelCleaning.* into shenandoah
subdirectory, so that they don't get compiled when Shenandoah is
excluded from build. They are only used by Shenandoah anyway (in
sh/jdk11).

http://cr.openjdk.java.net/~rkennke/isolate-markbitmap-parcleaning/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?