kprobe semaphore handling

[Stan Cox]

In dwarf_derived_probe_group::emit_module_init only setup the task_finder as 
needed:

+ s.op->newline() << "if (p->sdt_sem_offset) {";
   s.op->newline(1) << "rc = stap_register_task_finder_target(&p->finder);";
+ s.op->newline(-1) << "}";