hla_target: allow non-intrusive profiling on cortex-m

hla_target: allow non-intrusive profiling on cortex-m

Leverages the existing work that added profiling via DWT_PCSR.

hla_target doesn't have direct access to the mem_ap for doing a bulk
repeated read, but simply reading the DWT_PCSR register repeatedly is
still ~2 order of magnitude faster than halt/resume.

Change-Id: Ibe451aa95143694398370fdad6939cfb6191d56f
Signed-off-by: Karl Palsson <karlp@tweak.net.au>
Reviewed-on: http://openocd.zylin.com/4220
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>