gdb: or1k: add single step for linux native debugging

gdb: or1k: add single step for linux native debugging

Needed for single stepping in Linux, this adds the or1k implementation
of or1k_software_single_step.  Most of the implementation is borrowed
from the bare metal single step code from or1k_single_step_through_delay
which has been extracted and shared in helper function
or1k_delay_slot_p.