RFR [sh/8u]: Backport elimination of forwarding pointer

[Roman Kennke]

Notice that I backported jdk11's gc/shared/preservedMarks.* stuff into
gc_implementation/shenandoah for mark-compact.

Roman


> This backports the elimination of the forwarding pointer. It's the two
> changesets:
> 
> [backport] 8224584: Shenandoah: Eliminate forwarding pointer word
> [backport] 8223567: Rename ShenandoahBrooksPointer to ShenandoahForwarding
> 
> It also reverts a whole lot of shared code changes all over the place.
> 
> Testing: hotspot_gc_shenandoah (x86/aarch64), specjvm ongoing
> 
> Can I please get a review?
> 
> Thanks,
> Roman
>