package/gdb: switch to 8.2 as the default version

8.3 is around, 8.2 has already seen a point release, so it's time to
make 8.2 the default version for gdb.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
[Peter: also update BR2_GDB_VERSION logic]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>