tools/openocd-rtt: also open GDB port

When using OpenOCD RTT (real-time transfer) for stdio with
`USEMODULE=stdio_rtt make ...`, it is no longer possible to debug
while being connected to stdio. By also opening gdb at RIOT's default
GDB port, `make debug-client` can be used to connect from GDB to the
OpenOCD instance providing stdio via RTT.

dist/tools/openocd/openocd.sh

@@ -484,7 +484,7 @@ do_term() {
             -c 'bindto ${OPENOCD_SERVER_ADDRESS}' \
             -c 'tcl_port 0' \
             -c 'telnet_port 0' \
-            -c 'gdb_port 0' \
+            -c 'gdb_port 3333' \
             -c init \
             -c 'rtt setup '${RAM_START_ADDR}' '${RAM_LEN}' \"SEGGER RTT\"' \
             -c 'rtt start' \