RTS: Add missing memory barrier
In the work stealing queue a load-load-barrier is required to ensure
that a read of queue data cannot be reordered before a read of the
bottom pointer into the queue.
The added load-load-barrier ensures that the ordering of writes enforced
at the end of pushWSDeque is also respected in the order of reads in
stealWSDeque_. In other words, when reading q->bottom we want to make
sure that we see the updates to q->elements.
Fixes Trac #13633
(cherry picked from commit 5c084e0468be46f5ab48b2c1669a7e4d4d0f3c43)