ITS#7969 Use __sync_synchronize()

The rest of 9a80a8a8e8
plus commit a937740aa0.

libraries/liblmdb/mdb.c

@@ -3643,6 +3643,11 @@ mdb_env_write_meta(MDB_txn *txn)
 		mp->mm_dbs[0] = txn->mt_dbs[0];
 		mp->mm_dbs[1] = txn->mt_dbs[1];
 		mp->mm_last_pg = txn->mt_next_pgno - 1;
+#if (__GNUC__ * 100 + __GNUC_MINOR__ >= 404) && /* TODO: portability */	\
+	!(defined(__i386__) || defined(__x86_64__))
+		/* LY: issue a memory barrier, if not x86. ITS#7969 */
+		__sync_synchronize();
+#endif
 		mp->mm_txnid = txn->mt_txnid;
 		if (!(flags & (MDB_NOMETASYNC|MDB_NOSYNC))) {
 			unsigned meta_size = env->me_psize;