[SYCL-2020] Enabling USM support for SYCL. SYCL-1.2.1 did not have support for USM. As a result the Eigen SYCL has to mimic the USM style pointer via device pointer simulation. The proposed virtual pointer is an 8 bytes host pointer used as a key to access the SYCL device buffer. The reason was that the device buffer could not be used as a pointer to represent the m_data in Eigen leaf node expressions. Hence, the virtual pointer (that was the key in the buffer map) used in Eigen expression construction. This leads to the memory finding and looking in the buffer map to construct the Evaluator class. Using SYCL-2020 USM pointer allow us to remove all the unnecessary steps added to mimic the pointer for the device code in the SYCL backend.