Improve get_global_id() usage and coverage in OpenCL CTS

A couple of issues wrt current usage and coverage of get_global_id() and related functions -

- Most of the CTS code use int to store results of get_global_id that returns size_t
- CTS doesn't seem to have coverage for large values exceeding 32-bit range.

Start by auditing the OpenCL CTS code paths that use get_global_id() and related functions, checking how their results are stored. Add coverage for values exceeding the 32-bit range and verify that results are handled as size_t; done means the identified usage issues are corrected and the new coverage passes.

Written by the indexing model from the issue text.