sh: fix zImage build with >=binutils-2.18

Starting with binutils somewhere around 2.17.50.14 the vmlinux file
contains a ".note.gnu.build-id" section which doesn't get removed when
the zImage is built; resulting in a 2GB intermediate file and a broken
zImage.

Signed-off-by: Manuel Lauss <mano@roarinelk.homelinux.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>