Update DeallocationStack for Windows context switch

[HertzDevil]

Calling LibC.GetCurrentThreadStackLimits outside the main fiber of a thread will continue to return the original stack top of that thread:

LibC.GetCurrentThreadStackLimits(out low_limit, out high_limit)
low_limit  # => 86767566848
high_limit # => 86775955456

spawn do
  LibC.GetCurrentThreadStackLimits(out low_limit2, out high_limit2)
  low_limit2  # => 86767566848
  high_limit2 # => 1863570554880
end

sleep 1.second

This doesn't affect the Crystal runtime because the function is only called once to initialize the main thread, nor the GC because it probes the stack top using VirtualQuery. It may nonetheless affect other external libraries because the bad stack will most certainly contain unmapped memory.

The correct way is to also update the Win32-specific, non-NT DeallocationStack field, since this is the actual stack top that gets returned (see also Wine's implementation).