make -j zImage may call if_changed twice at the same time, the result is a
corrupted vmlinux.gz

Write to a temporary file for the time being until someone with make skills
fix the serialization properly.
Signed-off-by: Olaf Hering <olh@suse.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Tom Rini <trini@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>