uatomic/x86: Remove redundant memory barriers

uatomic/x86: Remove redundant memory barriers

When liburcu is configured to _not_ use atomic builtins, the
implementation of atomic operations is done using inline assembler for
each architecture.

Because we control the emitted assembler, we know whether specific
operations (e.g. lock; cmpxchg) already have an implicit memory barrier.
In those cases, emitting an explicit cmm_smp_mb() before/after the
operation is redundant and hurts performance.

Remove those redundant barriers on x86.

Change-Id: Ic1f6cfe9c2afe250946549cf6187f8fa88f5b009
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>