Run gdb as a subprocess, not execv'd

If ade execv's gdb then its process will be replaced, potentially
causing the ssh connection to the target device to fail which will kill
the remote gdb server. So instead run gdb in a subprocess, keeping ade
(and thus the ssh connection) alive during the debugging session.

Also instead of using gdb, use gdb-multiarch which is a variant capable
of interacting with gdbservers of multiple architectures (and
understand/parsing the debug symbols of multiple architectures). This
means it can be used to debug both ARM and Intel targets, rather then
just native targets.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5305