[PATCH] kbuild: allow cscope to index multiple architectures

I have a single source tree which I cross compile for a couple of
different architectures using ARHC=foo O=blah etc.

The existing cscope target is very handy but only indexes the current
$(ARCH), which is a pain since inevitably I'm interested in the other
one at any given time ;-). This patch allows me to pass a list of
architectures for cscope to index. e.g.
	make ALLSOURCE_ARCHS="i386 arm" cscope

This change also works for etags etc, and I presume it is just as useful
there.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>