target/semihosting: Fix of close(): Never close standard streams

target/semihosting: Fix of close(): Never close standard streams

This change fixes behavior of the SEMIHOSTING_SYS_CLOSE operation.
It ensures that OpenOCD's own stdin/stdout/stderr streams are never
closed, not even if the target requests it via semihosting.

Change-Id: Ia85af5963d1a3516284fd834f7197369a8fb268c
Signed-off-by: Jan Matyas <matyas@codasip.com>
Reviewed-on: http://openocd.zylin.com/6291
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>