x86,kgdb: Add low level debug hook

The only way the debugger can handle a trap in inside rcu_lock,
notify_die, or atomic_notifier_call_chain without a triple fault is
to have a low level "first opportunity handler" in the int3 exception
handler.

Generally this will be something the vast majority of folks will not
need, but for those who need it, it is added as a kernel .config
option called KGDB_LOW_LEVEL_TRAP.

CC: Ingo Molnar <mingo@elte.hu>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: H. Peter Anvin <hpa@zytor.com>
CC: x86@kernel.org
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>

kernel/debug/debug_core.c

@@ -66,7 +66,7 @@ int				kgdb_connected;
 EXPORT_SYMBOL_GPL(kgdb_connected);
 
 /* All the KGDB handlers are installed */
-static int			kgdb_io_module_registered;
+int			kgdb_io_module_registered;
 
 /* Guard for recursive entry */
 static int			exception_level;