Support serializing and unserializing in the O3 CPU. Also a few small fixes for...

Support serializing and unserializing in the O3 CPU.  Also a few small fixes for draining/switching CPUs.

src/cpu/o3/commit_impl.hh:
    Fix to clear drainPending variable on call to resume.
src/cpu/o3/cpu.cc:
src/cpu/o3/cpu.hh:
    Support serializing and unserializing in the O3 CPU.
src/cpu/o3/lsq_impl.hh:
    Be sure to say we have no stores to write back if the active thread list is empty.
src/cpu/simple_thread.cc:
src/cpu/simple_thread.hh:
    Slightly change how SimpleThread is used to copy from other ThreadContexts.

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extra : convert_revision : 92a5109b3783a989d5b451036061ef82c56d3121