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

arch/SConscript

@@ -70,7 +70,7 @@ def gen_switch_hdr_string(target, source, env):
     return "Generating ISA switch header " + str(target[0])
 
 # Build SCons Action object. 'varlist' specifies env vars that this
-# action depdnds on; when env['ALL_ISA_LIST'] changes these actions
+# action depends on; when env['ALL_ISA_LIST'] changes these actions
 # should get re-executed.
 switch_hdr_action = Action(gen_switch_hdr, gen_switch_hdr_string,
                            varlist=['ALL_ISA_LIST'])