[PATCH] Kbuild: index asm-$(SUBARCH) headers for UML

In Uml, many definitions are borrowed from underlying subarch headers (with
#include <asm/arch/stuff.h>).  And it has become annoying to keep switching
tag files all time, so by default index the underlying subarch headers too.
Btw, it adds negligible space to the tags file (less than 1M surely, IIRC it
was around 500k over 40M).

Finally, preserve the ALLSOURCE_ARCHS command line option (I hope) - if it is
set, it is used for headers too as before.  But check my construct please, I
didn't test this.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Acked-by: Jeff Dike <jdike@addtoit.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>