Global device symbol management proposal.

[Naghasan]

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Some devices can manage global values to push and pull different values. In the CUDA runtime, this is covered by cudaMemcpyToSymbol/cudaMemcpyFromSymbol functions. But other APIs may offer similar functionalities.

This proposal add the possibilities to manage such global values via a new class and new sycl::handler member functions.

The proposal is still a bit incomplete as regular C++ global variable can potentially be handled by the sycl::handler extension without requiring the new 'sycl::symbol' class. but limitation have yet to be explored.