GDB fileIO stdout support

GDB fileIO stdout support

This patch fixes gdb fileio support to allow gdb console to be used as stdout.

Now we can do something like
gdb <inferior file>

(gdb) tar ext :3333
(gdb) load
(gdb) monitor arm semihosting enable
(gdb) monitor arm semihosting_fileio enable
(gdb) continue

Here: Output from inferior using puts, printf etc will be routed to gdb console.

Change-Id: I9cb0dddda1de58038c84f5b035c38229828cd744
Signed-off-by: Omair Javaid <omair.javaid@linaro.org>
Reviewed-on: http://openocd.zylin.com/4538
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>