Fix Semihosting FileIO for targets using vcont packet

Fix Semihosting FileIO for targets using vcont packet

This patch fixes a bug where target fails to resume after completing GDB FileIO.
We need to update target last run control information to decide resumption. This
was not being done for vcont packets.

Change-Id: I44bea31720f8b877dba97d77a202303d546ea5bd
Signed-off-by: Omair Javaid <omair.javaid@linaro.org>
Reviewed-on: http://openocd.zylin.com/4539
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>