Enable building only selected CPU models via new scons

Enable building only selected CPU models via new scons
CPU_MODELS parameter.  For example:
scons CPU_MODELS="SimpleCPU,FullCPU" ALPHA_SE/m5.debug
Unfortunately the option is not sticky due to a scons
bug with saving & restoring ListOption parameters.

SConscript:
    Separate out cpu-model-specific files so they can be conditionally
    included based on value of new CPU_MODELS parameter.
    Most of these are now handled in cpu/SConscript, except for FullCPU
    which is still in this file.
arch/SConscript:
    The set of CPU-model-specific execute files must now be
    determined from the CPU_MODELS parameter, via the new
    cpu_models.py file.
    Also pass the list of configured CPU models to isa_parser.py.
arch/isa_parser.py:
    Move CpuModel definition and objects out to a
    separate file so they can be shared with scons.
    Global list of CPU models to generate code for is now
    controlled by command-line parameters (so we can do
    only a subset of the available ones).
build/SConstruct:
    Define new CPU_MODELS ListOption.
cpu/static_inst.hh:
    Rename static_inst_impl.hh to static_inst_exec_sigs.hh.

--HG--
extra : convert_revision : 163df32a76d4c05900490b2bce4c7962a5e3f614