AArch64: Use long for pointers in VelocityTracker class

For storing pointers, long is used in VelocityTracker class,
as native pointers can be 64-bit.

Change-Id: I8c454663a97745c7440bb6f99ef49d28e9026876
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>