If you kill the inferior from GDB we end up leaving our socket lying
around. Fix this by calling gdb_exit() first.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210108224256.2321-10-alex.bennee@linaro.org>