Fix generic atomic ops and/or: add missing return

Fixes build on powerpc:

./urcu/uatomic/generic.h: In function '_uatomic_and':
./urcu/uatomic/generic.h:310:2: warning: 'return' with a value, in
function returning void
./urcu/uatomic/generic.h: In function '_uatomic_or':
./urcu/uatomic/generic.h:374:2: warning: 'return' with a value, in
function returning void

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