Simplify the Windows implementation of CV_XADD

`_InterlockedExchangeAdd` is a Visual Studio intrinsic that's available for all architectures and in all VS versions that we care about. It's also potentially faster than the underscore-less function, since it's an intrinsic. We also don't need to declare it ourselves.

It is, however, a Visual Studio-specific intrinsic, so I changed the preprocessing condition accordingly.

Fixes <http://code.opencv.org/issues/3365>.