configs: Add --wait-gdb as option to se.py

I switch between waiting and non-waiting scenario many times per day.
The BaseCPU.wait_for_remote_gdb attribute, introduced in c2baaab0ed,
makes it much less painful by saving many recompiles.
The present commit tries to go a bit further: the se.py script is
under version control, and changing it interferes with smooth git
workflow.

Change-Id: Ie65ffc44b11d78d5e7878f81f2fcdafa143c20a8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27287
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

configs/example/se.py

@@ -275,5 +275,9 @@ else:
     MemConfig.config_mem(options, system)
     config_filesystem(system, options)
 
+if options.wait_gdb:
+    for cpu in system.cpu:
+        cpu.wait_for_remote_gdb = True
+
 root = Root(full_system = False, system = system)
 Simulation.run(options, root, system, FutureClass)