handle pre-OpenCL 1.2 ICDs

[bashbaug]

Currently, the OpenCL Runtime Loader checks for platforms that support the ICD extension by calling clGetExtensionFunctionAddressForPlatform, which is an OpenCL 1.2 function. Consider:

• Check for OpenCL 1.2 before calling clGetExtensionFunctionAddressForPlatform, in which case only OpenCL 1.2 and newer platforms will be enumerated.
• Check for the ICD extension in the platform extension string.