[MIPS] Make unwind_stack() can dig into interrupted context

If the PC was ret_from_irq or ret_from_exception, there will be no
more normal stackframe.  Instead of stopping the unwinding, use PC and
RA saved by an exception handler to continue unwinding into the
interrupted context.  This also simplifies the CONFIG_STACKTRACE code.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>