package/gdb: remove gdb 8.0.x

8.3 is around and 8.2 is the default version, so drop
8.0. Keep 7.12 which is the latest version that doesn't
requires C++11 support.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>