Pass IO::WRITABLE, not FMODE_WRITABLE, to the scheduler's io_wait - #9733

ioWaitReadable and ioWaitWritable passed OpenFile's mode flags as the
event mask. READABLE happens to match IO::READABLE, but WRITABLE is 2,
which is IO::PRIORITY, so a write that hit EAGAIN asked the scheduler
to wait for priority data. io-event's select backend then never put the
IO in the writable set and the writer hung.

Use the IO event values like MRI's rb_fiber_scheduler_io_wait_writable.