gdb: deprecate version 7.7.x

Now that 7.9 was added and 7.8.x is default we don't want to have a
dozen versions so mark it as deprecated.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>