[libomptarget] Factor architecture dependent code out of loop.cu
Related to the patch series starting D64217. Added subscribers to said series as reviewers. This effort is smaller in scope.
This patch factors out just enough architecture dependent code from loop.cu to allow the same source can be used with amdgcn, given a different target_impl.h. Testing is that the same bitcode (modulo variable names) is generated for libomptarget before and after the refactor, for nvptx and the out of tree amdgcn.