Wait for the GDB server to be listening before doing anything else

We were trying to connect the GDB client even before we got the
listening event. The debugger server thread is now looping forever.

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5327