Possibly missing OpenCL C enum: CLK_UNORM_INT_101010_2

[bashbaug]

OpenCL 2.1 added the image channel order CL_UNORM_INT_101010_2, but because we never updated the OpenCL C spec for OpenCL 2.1 we never added the corresponding kernel enum CLK_UNORM_INT_101010_2. I think we either need to add this enum to OpenCL C 3.0, or we need to define what other enum should be returned by get_image_channel_order() for a CL_UNORM_INT_101010_2 image.

This is partially described by #183, but the enum isn't described specifically.