osc/sm: do not use gcc specific calls

cmr=v1.7.5:reviewer=jsquyres

This commit was SVN r30910.

ompi/mca/osc/sm/osc_sm_passive_target.c

@@ -48,7 +48,7 @@ lk_fetch32(ompi_osc_sm_module_t *module,
            int target,
            size_t offset)
 {
-    __sync_synchronize();
+    opal_atomic_mb ();
     return (uint32_t) *((char*) &module->node_states[target].lock + offset);
 }