thread_pthread.c (rb_thread_create_mjit): set detach before create

This should be slightly cheaper on NPTL as it does not rely on
atomics to set pd->joinid.   We already use pthread_attr_setdetachstate,
so it won't introduce new problems by using a function we did not
use before.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62475 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread_pthread.c

@@ -1766,10 +1766,10 @@ rb_thread_create_mjit_thread(void (*child_hook)(void), void (*worker_func)(void)
 
     pthread_atfork(NULL, NULL, child_hook);
     if (pthread_attr_init(&attr) == 0
+        /* jit_worker thread is not to be joined */
+        && pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED) == 0
         && pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM) == 0
         && pthread_create(&worker_pid, &attr, mjit_worker, (void *)worker_func) == 0) {
-        /* jit_worker thread is not to be joined */
-        pthread_detach(worker_pid);
         return TRUE;
     }
     else {