target/semihosting: Capture errno in SEMIHOSTING_SYS_ISTTY

Capture the value of "errno" in semihosting isatty() call,
as is done in other syscalls.

Change-Id: I41b72175635f06c000536f583e3efa30fb57379e
Signed-off-by: Jan Matyas <matyas@codasip.com>
Reviewed-on: http://openocd.zylin.com/6292
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/semihosting_common.c

@@ -615,6 +615,7 @@ int semihosting_common(struct target *target)
 					return retval;
 				int fd = semihosting_get_field(target, 0, fields);
 				semihosting->result = isatty(fd);
+				semihosting->sys_errno = errno;
 				LOG_DEBUG("isatty(%d)=%d", fd, (int)semihosting->result);
 			}
 			break;