buildroot

gdb: switch to 7.7 as the default version

Now that gdb 7.8 is out, it's time to move to 7.7 as the default
version instead of 7.6.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>