[libomptarget] Build cuda plugin without cuda installed locally

[libomptarget] Build cuda plugin without cuda installed locally

Compiles a new file, `plugins/cuda/dynamic_cuda/cuda.cpp`, to an object file that exposes the same symbols that the plugin presently uses from libcuda. The object file contains dlopen of libcuda and cached dlsym calls. Also provides a cuda.h containing the subset that is used.

This lets the cmake file choose between the system cuda and a dlopen shim, with no changes to rtl.cpp.

The corresponding change to amdgpu is postponed until after a refactor of the plugin to reduce the size of the hsa.h stub required

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D95155