[MIPS] Simplify dump_stack()

Make dump_stack() code not depend on CONFIG_KALLSYMS.

It also make prepare_frametrace() always inlined to get
less false entries reported by show_raw_backtrace().

Signed-off-by: Franck Bui-Huu <vagabon.xyz@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>