kbuild: add GNU GLOBAL tags generation

GNU GLOBAL (http://www.gnu.org/software/global/

) is a source code tagging system
It is really cheap to support it in kbuild system.

Signed-off-by: Jianbin Kang <kjbmail@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>