rtos: Add support for Zephyr RTOS

rtos: Add support for Zephyr RTOS

With this patch, the Zephyr[1] RTOS is supported by OpenOCD.

As usual with support for other RTOSes, Zephyr must be compiled with
the DEBUG_THREAD_INFO option. This will generate some symbols
with information needed in order to build the list of threads.

The current implementation is limited to Zephyr running on ARM
Cortex-M processors. This is the only ARM variant supported by Zephyr
at the moment and is used on most of the officially supported boards.

[1] https://www.zephyrproject.org/

Change-Id: I22afdbec91562f3a22cf5b88cd4ea3a7a59ba0b4
Signed-off-by: Evgeniy Didin <didin@synopsys.com>
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>
Signed-off-by: Daniel Glöckner <dg@emlix.com>
Reviewed-on: http://openocd.zylin.com/4988
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>