kgdb,blackfin: Add in kgdb_arch_set_pc for blackfin

The new debug core api requires all architectures that use to debug
core to implement a function to set the program counter.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Acked-by: Sonic Zhang <sonic.zhang@analog.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>