Fix GCC7 warnings about switch-case fallthroughs

GCC7 with -Wextra warns about switch-case blocks which fallthrough with
"this statement may fall through [-Werror=implicit-fallthrough=]". This
can be fixed by adding "special" comments: "/* fallthrough */".

See https://gcc.gnu.org/gcc-7/changes.html

Change-Id: Iba0be791dbdd86984489b2d9a0592bb59828da1e
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/4174
Tested-by: jenkins

src/target/target.c

@@ -3684,7 +3684,7 @@ COMMAND_HANDLER(handle_bp_command)
 				addr = 0;
 				return handle_bp_command_set(CMD_CTX, addr, asid, length, hw);
 			}
-
+			/* fallthrough */
 		case 4:
 			hw = BKPT_HARD;
 			COMMAND_PARSE_ADDRESS(CMD_ARGV[0], addr);