Do a better job of factoring out CPU model in ISA description.

Do a better job of factoring out CPU model in ISA description.
(Still not perfect though.)

arch/alpha/isa_desc:
    Do a better job of factoring out CPU model.  (Still not perfect though.)
    Pull execute() methods out of class declarations into separate section
    of file, allowing (1) easier replication for different CPU models and
    (2) a path to putting them all in a separate file.  Force all instruction
    execution context into a single model-dependent class (SimpleCPU itself
    for SimpleCPU, DynInst for FullCPU).
arch/isa_parser.py:
    Do a better job of factoring out CPU model.  (Still not perfect though.)
    Pull execute() methods out of class declarations into separate section
    of file, allowing (1) easier replication for different CPU models and
    (2) a path to putting them all in a separate file.
    Also restructure top level to allow parser to run under interactive
    interpreter session for easier debugging.
cpu/exec_context.hh:
    Add a few new methods to clean up isa_desc.
cpu/simple_cpu/simple_cpu.cc:
cpu/static_inst.hh:
    StaticInst::execute no longer takes a CPU and an ExecContext,
    just a unified FooCPUExecContext.
cpu/simple_cpu/simple_cpu.hh:
    Add methods to redirect calls to ExecContext so SimpleCPU
    can act as sole instruction execution context for itself.
    Typedef SimpleCPU to SimpleCPUExecContext.

--HG--
extra : convert_revision : ecc445503bc585585da5663fe61796580e744da6