kbuild: Add support for installing generated asm headers

Generated asm headers are supposed to live in
arch/*/include/generated/asm, but objhdr-y expect them to live in the
same directory they are generated in.  Instead of trying to cut that
particular Gordian knot, introduce genhdr-y that takes this into
account; the sole user of objhdr-y, linux/version.h, should be
migrated over at some later date.

Suggested-by: David Woodhouse <dwmw2@infradead.org>
Acked-by: David Woodhouse <dwmw2@infradead.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>