Fix exported target for `vtkm::openmp` pointing to nonexistent files in MSYS2

Using ${OpenMP_CXX_LIBRARIES} results in paths to certain libraries being hard-coded in VTKmTargets.cmake, which can lead to issues in some cases (e.g.: when VTK is built by the CI system for MSYS2). This MR uses target OpenMP::OpenMP_CXX instead.

The second commit rewrites the logic for OpenMP to more closely match the code for TBB (calls to set_property are replaced by equivalents calls and moved).

Fixes #812 (closed) (tested locally)