scripts/tags.sh: Support subarch for ARM

Current tags.sh doesn't handle subarch for ARM.
There are too many subarch on ARM, it is hard that we locate some
functions which are defined in every subarch with tags util family.
Therefore support subarch for removing this unconvenience.

We can use ARM subarch functionality like below.
"make cscope O=. SRCARCH=arm SUBARCH=xxx"

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>