Introduce testing of core spec struct ABI stability

Testing of ABI stability should be automated, e.g. as for [sycl::exception](https://github.com/intel/llvm/blob/5e8085d94fb9fea40416735857b4092d44547565/sycl/test/abi/layout_exception.cpp). This would remove the chances of human error and avoid accidental ABI breakages.