kbuild: reintroduce ALLSOURCE_ARCHS support for tags/cscope

This patch reintroduce the ALLSOURCE_ARCHS support for tags/TAGS/
cscope targets. The Kbuild previously has this feature, but after
moving the targets into scripts/tags.sh, ALLSOURCE_ARCHS disappears.

It's something like this:

	$ make ALLSOURCE_ARCHS="x86 mips arm" tags cscope

Signed-off-by: Jike Song <albcamus@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>