s390 uatomic arch fix

    In file included from urcu-static.h:40,
                     from urcu.c:35:
    ./urcu/uatomic_arch.h: In function ‘_uatomic_exchange’:
    ./urcu/uatomic_arch.h:88: warning: dereferencing ‘void *’ pointer
    ./urcu/uatomic_arch.h:89: warning: dereferencing ‘void *’ pointer
    ./urcu/uatomic_arch.h:89: error: expected ‘:’ or ‘)’ before ‘(’ token
    ./urcu/uatomic_arch.h: In function ‘_uatomic_cmpxchg’:
    ./urcu/uatomic_arch.h:131: warning: dereferencing ‘void *’ pointer
    ./urcu/uatomic_arch.h:132: warning: dereferencing ‘void *’ pointer
    ./urcu/uatomic_arch.h:132: error: expected ‘:’ or ‘)’ before ‘(’ token

Use same hidden pointer technique used for x86.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>