RFR: Rename BrooksPointer to ShenandoahBrooksPointer

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/rename-brooksptr/webrev.01/

There is the convention to have Shenandoah-specific files and types to have "Shenandoah" in their
names. This changeset renames the major offender to that rule: BrooksPointer.

Testing: {x86_64, aarch64} builds, tier1_gc_shenandoah

Thanks,
-Aleksey