Fix InterlockedCompareExchange prototype

Another bug found when implementing gthreads in druntime.

See: http://msdn.microsoft.com/en-us/library/windows/desktop/ms683560%28v=vs.85%29.aspx

``InterlockedCompareExchange`` does not return PVOID, and does not accept the parameters ``(PVOID*, PVOID, PVOID)``

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