Support for debugging on ARMv8-M CPUs

Support for debugging on ARMv8-M CPUs

This patch adds ARMv8-M CPUs detection logic in ARMv7m target specific code.

Also adds a slightly different watchpoint manipulation logic for ARMv8-M.

This is based on ARMv8-M architecture reference manual.

Tested on ARM Musca A board.

Change-Id: I0652560954ef02c378a7067fab586edf39d3e9cc
Signed-off-by: Omair Javaid <omair.javaid@linaro.org>
Reviewed-on: http://openocd.zylin.com/4997
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>