[IA64] always build arch/ia64/lib/xor.o

Always build ia64 xor.o because multiple config options now depend on it.

Necessary to build .20-mm* on ia64 when, e.g., CONFIG_ASYNC_TX_DMA is
defined.  Don't know if '_ASYNC_TX_DMA makes sense on ia64.  If not, maybe
Kconfig should preclude it.

Could have defined a Kconfig option that defaults to true if MD_RAID456 ||
ASYNC_TX_DMA to control building of xor.o, but xor.o is only 848 bytes and
this IS ia64...

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Bob Picco <bob.picco@hp.com>
Cc: Eric Whitney <eric.whitney@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>