Blackfin arch: Fix bugs - Make kgdb code apparent to app debugging.

 - Skip single step if global interrupt disable bit is set.
 - Extend bernds' patch r4673 to skip single step in any interrupt entry
   that interrupts the code which is under single stepping. Bernds' patch
   only allow user space single stepping.

Singed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>