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>

scripts/tags.sh

@@ -114,6 +114,11 @@ docscope()
 	cscope -b -f cscope.out
 }
 
+dogtags()
+{
+	all_sources | gtags -f -
+}
+
 exuberant()
 {
 	all_sources | xargs $1 -a                               \
@@ -185,6 +190,10 @@ case "$1" in
 		docscope
 		;;
 
+	"gtags")
+		dogtags
+		;;
+
 	"tags")
 		rm -f tags
 		xtags ctags