Honor URCU_DEREFERENCE_USE_VOLATILE

Honor URCU_DEREFERENCE_USE_VOLATILE

When users define `URCU_DEREFERENCE_USE_VOLATILE', `rcu_dereference()'
is implemented using `CMM_ACCESS_ONCE()', even when compiled with atomic
builtins or if the compiler support the C11 memory model.

This allows optimization on weakly-ordered architectures where the
`CMM_CONSUME' memory order is implemented as `CMM_ACQUIRE'.

To prevent the compiler from doing certain optimizations on pointers
equivalence, introduce the `cmm_ptr_eq()' API for pointer comparisons.

This also introduces the `_CMM_OPTIMIZER_HIDE_VAR()' private macro for
forcing the compiler optimizer to believe that a variable can be
manipulated arbitrarily, thus preventing optimizations based on pointer
equivalence.

Change-Id: Ib48c29370017990e6acf40a7590ab286a89691bd
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>