kgdb,sh: update superh kgdb exception handling

Implement kgdb_arch_pc() which adjusts the pc if it needs to be
adjusted after a software breakpoint trap.

Implement kgdb_arch_set_pc() which is a new required function in the
kgdb debug core.

When processing a single step return zero in the error exception field
so that the debug core can distinguish between a single step trap and
a breakpoint trap generically.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Acked-by: Paul Mundt <lethal@linux-sh.org>