Implement KHR ICD extension

Implement KHR ICD extension

This adds a pointer to the dispatch table at the beginning of every object
of type

 - cl_command_queue
 - cl_context
 - cl_device_id
 - cl_event
 - cl_kernel
 - cl_mem
 - cl_platform_id
 - cl_program
 - cl_sampler

as required by the ICD specification. The layout of the dispatch table
comes from the OpenCL ICD loader by Brice Videau <brice.videau@imag.fr> and
Vincent Danjean <Vincent.Danjean@ens-lyon.org>.

To avoid dispatch table entries being overwritten with the ICD loader's
implementations of the CL functions (as would be the proper behaviour for
the ELF loader), the -Bsymbolic option is given to the linker.

Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>