target/arc: refactor ARC register numbers defines

target/arc: refactor ARC register numbers defines

For Zephyr rtos support it is necessary to define general register
numbers for architecture. There were some already in arc.h file.
Let's define ARC registers numbers as a set instead of separate defines.

Change-Id: I63742b8608f9556c2ec9bd2661a0fd9cf88e9b74
Signed-off-by: Evgeniy Didin <didin@synopsys.com>
Reviewed-on: http://openocd.zylin.com/6105
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>