thread_pthread.c (native_sleep): reduce ppoll sleeps

By holding into sigwait_fd until after we acquire GVL, we can
hit the faster native_cond_sleep path instead of ppoll when
another thread wants to start sleeping.  ppoll-ing on sigwait_fd
isn't really useful in program where GVL is contended

This also allows reducing vm->gvl.lock mutex contention on
waitpid sleep migrations.

                         r64170        this patch
vm_thread_condvar1        0.921        1.356

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64193 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread_pthread.c

@@ -1716,9 +1716,9 @@ native_sleep(rb_thread_t *th, struct timespec *timeout_rel)
             check_signals_nogvl(th, sigwait_fd);
         }
         unblock_function_clear(th);
+        GVL_UNLOCK_END(th);
         rb_sigwait_fd_put(th, sigwait_fd);
         rb_sigwait_fd_migrate(th->vm);
-        GVL_UNLOCK_END(th);
     }
     else {
         native_cond_sleep(th, timeout_rel);