Make cortex_m3 functions public accessible.

Change this functions to make it accessible for other
target implementations.

Change-Id: Ib41fc793cfb4de5439af026c2e8b52e7a9507c85
Signed-off-by: Mathias K <kesmtp@freenet.de>
Reviewed-on: http://openocd.zylin.com/278


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