RFR: JDK-8223567: 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.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8223567
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8223567/webrev.00/

Testing: hotspot_gc_shenandoah (x86,aarch64), also with and without 
--disable-precompiled-headers

Ok?

Roman