Clarify program scope variables with subdevices

The OpenCL C 2.0 spec says the following about program scope variables (paragraph 6.5.1):

"These variables in the global address space have the same lifetime as the program, and their values persist between calls to any of the kernels in the program. These variables are not shared across devices. They have distinct storage."

There is nothing specified with respect to whether program scope variables are shared or not between subdevices of the same root device.

This requires some clarification and probably associated conformance tests.

Start with OpenCL C 2.0 specification paragraph 6.5.1 and examine how program scope variable storage should behave across subdevices of one root device. The work is done when the specification explicitly states the sharing rule and associated conformance tests cover that behavior.

Written by the indexing model from the issue text.