8008079: G1: Add nextObject routine to CMBitMapRO and replace nextWord Summary: Update the task local finger to the start of the next object when marking aborts, in order to avoid the redundant scanning of all 0's when the marking task restarts, if otherwise updating to the next word. In addition, reuse the routine nextObject() in routine iterate(). Reviewed-by: johnc, ysr Contributed-by: tamao <tao.mao@oracle.com>