Make FullCPU schedule its TickEvent when one of its contexts becomes active.

Make FullCPU schedule its TickEvent when one of its contexts becomes active.
This fixes detailed-mpboot, which was broken as of my last change.
Also clean up some of the ExecContext status initialization.

cpu/base_cpu.hh:
    CPU::execCtxStatusChg() now takes thread_num as an arg so the CPU knows
    which execContext had the status change.
    BaseCPU::registerExecContexts() no longer needs to be virtual.
cpu/exec_context.cc:
    Initialize _status directly... don't use setStatus() as this will notify the CPU
    of the change before it is ready.
    CPU::execCtxStatusChg() now takes thread_num as an arg so the CPU knows
    which execContext had the status change.
cpu/exec_context.hh:
    Don't need initStatus() any more.
cpu/simple_cpu/simple_cpu.cc:
    Move execCtxStatusChg() from header to .cc file.
    No longer need specialized version of registerExecContexts to schedule TickEvent.
cpu/simple_cpu/simple_cpu.hh:
    Move execCtxStatusChg() from header to .cc file.
    CPU::execCtxStatusChg() now takes thread_num as arg (must be 0 for SimpleCPU).
    No longer need specialized version of registerExecContexts to schedule TickEvent.
kern/tru64/tru64_system.cc:
    Don't need initRegs; the PC etc. get initialized in the CPU constructor.
    ExecContexts start out as Unallocated, so no need to set them to Unallocated here.
kern/tru64/tru64_system.hh:
    Don't need initRegs; the PC etc. get initialized in the CPU constructor.
sim/prog.cc:
    ExecContexts start out as Unallocated, so no need to set them to Unallocated here.

--HG--
extra : convert_revision : e960ebbeb845960344633798e251b6c8bf1c0378