run: add --terminal and explain gem5 pdb

run

@@ -123,6 +123,14 @@ rare and don't affect performance, because `./configure
 `pr_manager_run`, and I don't know how to get rid of them.
 """
 )
+init_group.add_argument(
+    '--terminal', default=False, action='store_true',
+    help='''Output to the terminal, don't pipe to tee as the default.
+Does not save the output to a file, but allows you to use debuggers.
+Set automatically by --debug-vm, but you still need this option to debug
+gem5 Python scripts.
+'''
+)
 parser.add_argument(
     '-U', '--tmux-args', default='',
     help='Pass extra parameters to the program running on the `-u` tmux split'
@@ -374,7 +382,7 @@ if args.tmux:
         ])
 
 cmd += extra_emulator_args
-if debug_vm:
+if debug_vm or args.terminal:
     out_file = None
 else:
     out_file = common.termout_file