c6x: kernel: setup: Export symbol memory_end

It is needed by several modules, the related error with allmodconfig:

    MODPOST 3327 modules
  ERROR: "memory_end" [net/wireless/lib80211_crypt_tkip.ko] undefined!
  ERROR: "memory_end" [net/sunrpc/sunrpc.ko] undefined!
  ERROR: "memory_end" [net/sunrpc/auth_gss/rpcsec_gss_krb5.ko] undefined!
  ERROR: "memory_end" [net/rxrpc/rxkad.ko] undefined!
  ERROR: "memory_end" [net/mac802154/mac802154.ko] undefined!
  ERROR: "memory_end" [net/mac80211/mac80211.ko] undefined!
  ERROR: "memory_end" [net/ipv6/esp6.ko] undefined!
  ERROR: "memory_end" [net/ipv6/ah6.ko] undefined!
  ERROR: "memory_end" [net/ipv4/esp4.ko] undefined!
  ERROR: "memory_end" [net/ipv4/ah4.ko] undefined!
  ERROR: "memory_end" [net/ceph/libceph.ko] undefined!
  ERROR: "memory_end" [net/9p/9pnet_virtio.ko] undefined!
  ERROR: "memory_end" [drivers/usb/wusbcore/wusbcore.ko] undefined!
  ERROR: "memory_end" [drivers/usb/misc/usbtest.ko] undefined!
  ERROR: "memory_end" [drivers/usb/core/usbcore.ko] undefined!
  ERROR: "memory_end" [drivers/target/target_core_file.ko] undefined!
  ERROR: "memory_end" [drivers/net/wireless/brcm80211/brcmfmac/brcmfmac.ko] undefined!
  ERROR: "memory_end" [drivers/net/wireless/ath/ath6kl/ath6kl_sdio.ko] undefined!
  ERROR: "memory_end" [drivers/net/virtio_net.ko] undefined!
  ERROR: "memory_end" [drivers/net/usb/usbnet.ko] undefined!
  ERROR: "memory_end" [drivers/net/ppp/ppp_mppe.ko] undefined!
  ERROR: "memory_end" [drivers/mtd/nand/nand.ko] undefined!
  ERROR: "memory_end" [drivers/mmc/card/mmc_block.ko] undefined!
  ERROR: "memory_end" [drivers/crypto/qce/qcrypto.ko] undefined!
  ERROR: "memory_end" [drivers/block/drbd/drbd.ko] undefined!
  ERROR: "memory_end" [drivers/block/aoe/aoe.ko] undefined!
  ERROR: "memory_end" [crypto/tcrypt.ko] undefined!
  ERROR: "memory_end" [crypto/gcm.ko] undefined!
  ERROR: "memory_end" [crypto/cts.ko] undefined!
  ERROR: "memory_end" [crypto/ccm.ko] undefined!
  ERROR: "memory_end" [crypto/authencesn.ko] undefined!
  ERROR: "memory_end" [crypto/authenc.ko] undefined!

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Mark Salter <msalter@redhat.com>