Add helper to catch single step debug event and distinguish it from bkpt

Some architectures (including ARMv6/v7) do not have separate single step
events and cannot see difference between breakpoint and single step.
Add db_pc_is_singlestep() to avoid skipping instruction we stepped on
to trigger debug event.
This commit does not change the existing functionality but adds possibility
to implement custom db_pc_is_singlestep().

Reviewed by: imp
Submitted by: Zbigniew Bodek <zbb@semihalf.com>
Obtained from: Semihalf
Sponsored by: Juniper Networks Inc.
Differential Revision: https://reviews.freebsd.org/D4036