[IA64] define "_sdata" symbol

core_kernel_data() wants to know if an address looks like kernel
data. IA64 has had _edata forever, but never needed _sdata until
now.
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/kernel/vmlinux.lds.S

@@ -209,6 +209,7 @@ SECTIONS {
 	data : {
 	} :data
 	.data : AT(ADDR(.data) - LOAD_OFFSET) {
+		_sdata  =  .;
 		INIT_TASK_DATA(PAGE_SIZE)
 		CACHELINE_ALIGNED_DATA(SMP_CACHE_BYTES)
 		READ_MOSTLY_DATA(SMP_CACHE_BYTES)