Blackfin arch: Fix bug - skip single step in high priority interrupt handler...

Blackfin arch: Fix bug - skip single step in high priority interrupt handler instead of disabling all interrupts in single step debugging.

Skip single step if event priority of current instruction is higher than
that of the first instruction, from which gdb starts single step.

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