ck_epoch: Specialize begin according to memory model.

Previously, we were specializing for x86.

include/ck_epoch.h

@@ -34,6 +34,7 @@
  */
 
 #include <ck_cc.h>
+#include <ck_md.h>
 #include <ck_pr.h>
 #include <ck_stack.h>
 #include <stdbool.h>
@@ -101,7 +102,7 @@ ck_epoch_begin(ck_epoch_t *epoch, ck_epoch_record_t *record)
 		 */
 		ck_pr_store_uint(&record->epoch, g_epoch);
 
-#if defined(__x86__) || defined(__x86_64__)
+#if defined(CK_MD_TSO)
 		ck_pr_fas_uint(&record->active, 1);
 		ck_pr_fence_atomic_load();
 #else