Fix RUBY_MN_THREADS sleep returning prematurely (ruby#15868)

timer_thread_check_exceed() was returning true when the remaining time
was less than 1ms, treating it as "too short time". This caused
sub-millisecond sleeps (like sleep(0.0001)) to return immediately
instead of actually sleeping.

The fix removes this optimization that was incorrectly short-circuiting
short sleep durations. Now the timeout is only considered exceeded when
the actual deadline has passed.

Note: There's still a separate performance issue where MN_THREADS mode
is slower for sub-millisecond sleeps due to the timer thread using
millisecond-resolution polling. This will require a separate fix to
use sub-millisecond timeouts in kqueue/epoll.

[Bug #21836]

Author: khasinski

test/ruby/test_thread.rb

@@ -1652,4 +1652,16 @@ def test_mutexes_locked_in_fiber_dont_have_aba_issue_with_new_fibers
       end
     end;
   end
+
+  # [Bug #21836]
+  def test_mn_threads_sub_millisecond_sleep
+    assert_separately([{'RUBY_MN_THREADS' => '1'}], "#{<<~"begin;"}\n#{<<~'end;'}", timeout: 30)
+    begin;
+      t0 = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+      1000.times { sleep 0.0001 }
+      t1 = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+      elapsed = t1 - t0
+      assert_operator elapsed, :>=, 0.1, "sub-millisecond sleeps should not return immediately"
+    end;
+  end
 end

thread_pthread.c

@@ -2947,15 +2947,7 @@ timer_thread_check_signal(rb_vm_t *vm)
 static bool
 timer_thread_check_exceed(rb_hrtime_t abs, rb_hrtime_t now)
 {
-    if (abs < now) {
-        return true;
-    }
-    else if (abs - now < RB_HRTIME_PER_MSEC) {
-        return true; // too short time
-    }
-    else {
-        return false;
-    }
+    return abs <= now;
 }
 
 static rb_thread_t *