cortex_m: add profiling function

Use DWT_PCSR if present (reads nonzero); otherwise do halt-and-sample pc.

Signed-off-by: Simon Schubert <2@0x2c.org>
Change-Id: Id2dc4665e5008cc497a2e6e6493522d038d5af42
Reviewed-on: http://openocd.zylin.com/4211
Tested-by: jenkins
Reviewed-by: Karl Palsson <karlp@tweak.net.au>
Reviewed-by: Christopher Head <chead@zaber.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>