target: added events TARGET_EVENT_STEP_START and _END

target: added events TARGET_EVENT_STEP_START and _END

Events TARGET_EVENT_STEP_START and TARGET_EVENT_STEP_END
have been added - analogous to already existing events
TARGET_EVENT_RESUME_*.

This is an example of a concrete use case where having
these events is important:

In RISC-V processors without Debug Program Buffer, OpenOCD
cannot execute fence/fence.i when resuming or single-
stepping. With these events implemented, the user can
instead provide custom operations to achieve that same
effect prior to resuming the processor.

Change-Id: I786348ff08940759d99b0f24e9e0ed5a44581094
Signed-off-by: Jan Matyas <matyas@codasip.com>
Reviewed-on: http://openocd.zylin.com/5551
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>