[MIPS] Rearrange show_stack, show_trace

Print call-trace in show_stack() (like on other archs).  Also make
show_trace() static and simplify its argument list.

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