Supported an OPENCL_VISIBLE_DEVICES environment variable inspired fro…

[cdai2]

…m CUDA_VISIBLE_DEVICES.

OPENCL_VISIBLE_DEVICES=descriptor_list descriptor_list -> descriptor | descriptor::descriptor_list descriptor -> driver,platform | driver,platform,type | driver,platform,type,device_list device_list -> device_id | device_id,device_list driver: client driver (foo.icd or foo.so) platform: platform ID type: gpu, cpu, accelerator, custom, or any device_id: device ID in the platform

This patch is to land https://github.com/KhronosGroup/OpenCL-ICD-Loader/pull/45 again. Most of the code is from pull request 45.

e.g., OPENCL_VISIBLE_DEVICES=foo.icd,0,gpu,0,1::bar.icd,1,cpu,0 only shows

• GPU 0, 1 in the first platform of foo.icd
• CPU 0 in the second platform of bar.icd