make activation of exec contexts happen in startup

make activation of exec contexts happen in startup
the registration stuff all moves into BaseCPU

cpu/base_cpu.cc:
    Move the registration stuff into the BaseCPU since all
    other CPUs use it.
cpu/base_cpu.hh:
    Move the defer registration stuff into the BaseCPU since all
    other CPUs use it.
cpu/simple_cpu/simple_cpu.cc:
cpu/simple_cpu/simple_cpu.hh:
    registration stuff moved to base class
sim/system.cc:
    the activation of exec contexts should happen at startup, not
    when they are registered.
sim/system.hh:
    the system now has a startup function

--HG--
extra : convert_revision : bb6a7c2da5a1ecf5fe7ede1078200bfe5245f8ef