kbuild: create symlink to vmlinux-gdb.py in scripts_gdb target

It is weird to create gdb stuff as a side-effect of vmlinux.

Move it to a more relevant place.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>