sh: Use the new stack unwinder API

Instead of implementing our own stack unwinder via dump_trace() we
should use the new stack unwinder API because it is more modular. This
change allows us to decouple the interface for generating stacktraces
from the implementation of a stack unwinder.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>