toolchain: use same gdb Config.in for internal/external toolchains

We used to use different gdb configs for internal and external toolchains
because mconf won't source the same file twice. This works, but is kind of
sub optimal, as people forget to keep them in sync.

Fix it to use the same file for both situations by shuffling around the
config options a bit. Should work identical to before (except for the newer
gdb versions available for ext).