Added functionality to the SYS_SYSTEM semihosting call.

There seems to be a few missing semihosting calls.  I
am not sure why this one is actually missing, since it
seems simple enough to implement.  It was tested using
an HTC HD7 connected to openocd through a "home brew"
ftdi 4232H board.

Change-Id: Ie17dc96c6d48227a3dc9ff1e21201a85498a10b1
Signed-off-by: Brandon Warhurst <roboknight@gmail.com>
Reviewed-on: http://openocd.zylin.com/1345
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>