Clarify requirements for the `std::size_t` and `std::ptrdiff_t` type aliases

The SYCL specification relies on the `std::size_t` and `std::ptrdiff_t` type aliases, but does not currently impose requirements that they be defined on the host and device to either the same type, or to types with matching size and alignment. Can objects of this type transit the host and device boundary or be accessed via USM? Since these types are used in the signatures of standard library functions, their exact definition has ABI implications.

This issue has been filed to update the SYCL specification to more explicitly state the intended requirements for these type aliases.