Add USM/SVM implementations for OpenCL, SYCL and OpenMP

[tomdeakin]

Add option for OpenCL 2.0 shared virtual memory buffers. This will allow measurement of the overhead of the implementations over the device interconnect.

[tomdeakin]

Would also be nice to add a CUDA UVM option.

[tomdeakin]

CUDA memory option added in cf42335

[tomdeakin]

We should add USM versions for:

• OpenCL
• SYCL
• OpenMP

[tom91136]

Progress:

• OpenCL
• SYCL (via v5.0)
• OpenMP