RFR: Rename ShenandoahBrooksPointer to ShenandoahForwarding

[Roman Kennke]

In preparation of elimination of the brooks pointer slot, I'd like to
rename ShenandoahBrooksPointer to ShenandoahForwarding. This name seems
more appropriate then. I also renamed some of the accessors to make more
sense in the new context.

http://cr.openjdk.java.net/~rkennke/renamebrooksptr/webrev.00/

Testing: hotspot_gc_shenandoah (x86,aarch64)

Ok?

Roman