mips,kgdb: kdb low level trap catch and stack trace

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

Also added was a die notification for oops such that kdb can catch an
oops for analysis.

There appeared to be no obvious way to pass the struct pt_regs from
the original exception back to the stack back tracer, so a special
case was added to show_stack() for when kdb is active because you
generally desire to generally look at the back trace of the original
exception.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/kgdb.h

@@ -38,6 +38,8 @@ extern int kgdb_early_setup;
 extern void *saved_vectors[32];
 extern void handle_exception(struct pt_regs *regs);
 extern void breakinst(void);
+extern int kgdb_ll_trap(int cmd, const char *str,
+			struct pt_regs *regs, long err, int trap, int sig);
 
 #endif				/* __KERNEL__ */