cpu: clean up architectural register classification

Move from a poorly documented scheme where the mapping
of unified architectural register indices to register
classes is hardcoded all over to one where there's an
enum for the register classes and a function that
encapsulates the mapping.

src/cpu/SConscript

@@ -118,6 +118,7 @@ Source('nativetrace.cc')
 Source('pc_event.cc')
 Source('profile.cc')
 Source('quiesce_event.cc')
+Source('reg_class.cc')
 Source('static_inst.cc')
 Source('simple_thread.cc')
 Source('thread_context.cc')