openrisc: Export symbols needed by modules

This was detected by allmodconfig, errors reported:

 ERROR: "empty_zero_page" [net/ceph/libceph.ko] undefined!
 ERROR: "__ucmpdi2" [lib/842/842_decompress.ko] undefined!
 ERROR: "empty_zero_page" [fs/nfs/objlayout/objlayoutdriver.ko] undefined!
 ERROR: "empty_zero_page" [fs/exofs/exofs.ko] undefined!
 ERROR: "empty_zero_page" [fs/crypto/fscrypto.ko] undefined!
 ERROR: "__ucmpdi2" [fs/btrfs/btrfs.ko] undefined!
 ERROR: "pm_power_off" [drivers/regulator/act8865-regulator.ko] undefined!
 ERROR: "__ucmpdi2" [drivers/media/i2c/adv7842.ko] undefined!
 ERROR: "__clear_user" [drivers/md/dm-mod.ko] undefined!
 ERROR: "__clear_user" [net/netfilter/x_tables.ko] undefined!

Signed-off-by: Stafford Horne <shorne@gmail.com>