kbuild: do not include arch/<ARCH>/include/asm in find-sources twice.

Architectures which have moved their includes to arch/<ARCH>/include
now list the headers twice in the source listing used by "make
cscope" and friends, causing those tools to list symbols twice.

Skipping these files in the ALLSOURCE_ARCHS pass rather than removing
the ALLINCLUDE_ARCHS pass preserves the semantics of the later.

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