Make sure cpu/static_inst_exec_sigs.hh get rebuilt when

Make sure cpu/static_inst_exec_sigs.hh get rebuilt when
CPU_MODELS parameter changes.

arch/SConscript:
    Fix typo in comment.
cpu/SConscript:
    Convert exec signature generator to Action so we can add dependency
    on CPU_MODELS environment var.
    Print nicer string while we're at it.
    Also add some comments.

--HG--
extra : convert_revision : bcb38a7941943cf071dac34cdbb2ece5456b8620