Fix preview_mt infinite loop on Windows

[HertzDevil]

After #13362, all programs on Windows will get stuck while initializing constants if -Dpreview_mt is provided:

• Crystal::OnceState#once calls Mutex#lock
• which calls Fiber.current
• which calls Crystal::Scheduler.current_fiber
• which calls Thread#scheduler
• which calls Crystal::Scheduler.new
• which initializes @fiber_channel by calling Crystal::FiberChannel.new
• which initializes @worker_out and @worker_in by calling IO.pipe
• which calls Crystal::System::FileDescriptor.pipe
• which now refers to constants yet to be initialized, calling Crystal::OnceState#once
• ...

This PR breaks the loop so that programs will at least continue to run with -Dpreview_mt (and work as long as no concurrency or multithreading features are used).