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

[Roman Kennke]

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