Issue about memory order on ARM

The code in GetSnapshotData() that read the `xid` field of  PGXACT struct has a dependency on code in GetNewTransactionId() that write `MyPgXact->xid`. It means that the store of xid should happen before the load of it. In C11, we can use [Release-Acquire ordering](https://en.cppreference.com/w/c/atomic/memory_order#Release-Acquire_ordering) to achieve it. But now, there is no special operation to do it(, and the [volatile](https://en.cppreference.com/w/c/language/volatile) keyword should not play any role in this situation).

So it means that when a backend A returns from GetNewTransactionId(), the newval of `MyPgXact-&gt;xid` maybe just in CPU store buffer, or CPU cache line, so the newval is not yet visible to backend B that calling GetSnapshotData(). So the assumption that 'all top-level XIDs <= latestCompletedXid are either present in the ProcArray, or not running anymore' may not be safe.&nbsp;