target/m68k: Add URL to semihosting spec

The spec for m68k semihosting is documented in the libgloss
sources. Add a comment with the URL for it, as we already
have for nios2 semihosting.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20230801154451.3505492-1-peter.maydell@linaro.org

target/m68k/m68k-semi.c

@@ -15,6 +15,10 @@
  *
  *  You should have received a copy of the GNU General Public License
  *  along with this program; if not, see <http://www.gnu.org/licenses/>.
+ *
+ *  The semihosting protocol implemented here is described in the
+ *  libgloss sources:
+ *  https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=libgloss/m68k/m68k-semi.txt;hb=HEAD
  */
 
 #include "qemu/osdep.h"