nds32: support FreeRTOS

Change-Id: I117b5541fb19388c0f5c2344ee42d9151c9a222e
Signed-off-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-on: http://openocd.zylin.com/1577


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>