Make the exit after max checkpoints code compile.
Make the exit after max checkpoints code compile.

sim/serialize.cc:
    call exitNow instead of SimExit.  Include the header too.

haowu4682/gem5,LingxiaoJIA/gem5,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,andrewfu0325/gem5-aladdin,haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5