gdb/cli: add a 'normal_stop' option to 'cli_suppress_notification'

Extend the 'cli_suppress_notification' struct with a new field,
'normal_stop', that can be used for checking if printing normal stop
events on the CLI should be suppressed.

This patch only introduces the flag.  The subsequent patch adds a user
command to turn the flag off/on.

gdb/command.h

@@ -379,6 +379,9 @@ struct cli_suppress_notification
 {
   /* Inferior, thread, frame selected notification suppressed?  */
   bool user_selected_context = false;
+
+  /* Normal stop event suppressed? */
+  bool normal_stop = false;
 };
 
 extern struct cli_suppress_notification cli_suppress_notification;