Blackfin arch: fix 2 bugs related to debug

 - unable to single step over emuexcpt instruction
 - gdbproxy goes into infinite loop when doing gdb does "next" over
   "emuexcpt"

Don't decrement PC after software breakpoint.

Signed-off-by: Jie Zhang <jie.zhang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>