Simplified 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
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>.
Merge pull request #1835 from SpecLad:windows-xadd