urcu-pointer: implement URCU_FORCE_CAST for C++ compatibility of urcu-pointer.h

urcu-pointer: implement URCU_FORCE_CAST for C++ compatibility of urcu-pointer.h

We need to be careful with those, so we do not break aliasing. Our
use-case is to cast back and forth between the same type and a void *
(or void **) type when we pass pointers to C functions. As we cast back
to the same type when the pointer is returned from the function,
aliasing should still work.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>