tags: Add the ability to make tags for all archs using "all"

make ALLSOURCE_ARCHS=all tags
- Document this in kbuild.txt
Without this change you have to type each arch separately.

Signed-off-by: John Kacur <jkacur@redhat.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>