scripts: Adopted better input file handling in result scripts

- Error on no/insufficient files.

  Instead of just returning no results. This is more useful when
  debugging complicated bash scripts.

- Use elf magic to allow any file order in perfbd.py/stack.py.

  This was already implemented in stack.py, now also adopted in
  perfbd.py.

  Elf files always start with the magic string "\x7fELF", so we can use
  this to figure out the types of input files without needing to rely on
  argument order.

  This is just one less thing to worry about when invoking these
  scripts.

scripts/code.py

@@ -1015,7 +1015,15 @@ def main(obj_paths, *,
         **args):
     # find sizes
     if not args.get('use', None):
+        # not enough info?
+        if not obj_paths:
+            print("error: no *.o files?",
+                    file=sys.stderr)
+            sys.exit(1)
+
+        # collect info
         results = collect(obj_paths, **args)
+
     else:
         results = []
         with openio(args['use']) as f: