sparc64: build compressed image (zImage) by default

Besides creating the uncompressed vmlinux image for sparc64, also
create a compressed zImage. This is more consistent with other
architectures and required to make the 'deb-pkg' target work.

Signed-off-by: Jurij Smakov <jurij@wooyd.org>
Signed-off-by: Frans Pop <elendil@planet.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>