lib/ovs-atomic: Add helpers for relaxed atomic access.

lib/ovs-atomic: Add helpers for relaxed atomic access.

When an atomic variable is not serving to synchronize threads about
the state of other (atomic or non-atomic) variables, no memory barrier
is needed with the atomic operation.  However, the default memory
order for an atomic operation is memory_order_seq_cst, which always
causes a system-wide locking of the memory bus and prevents both the
CPU and the compiler from reordering memory accesses accross the
atomic operation.  This can add considerable stalls as each atomic
operation (regardless of memory order) always includes a memory
access.

In most cases we can let the compiler reorder memory accesses to
minimize the time we spend waiting for the completion of the atomic
memory accesses by using the relaxed memory order.  This patch adds
helpers to make such accesses a little easier on the eye (and the
fingers :-), but does not try to hide them completely.

Following patches make use of these and remove all the (implied)
memory_order_seq_cst use from the OVS code base.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>