Add generic support for OpenCL
If Apple does jump ship to Arm then AMD won't be the only OpenCL fruit.

The Raspberry Pi seems to be moving towards an implementation of OpenCL (https://github.com/doe300/VC4CL).  

I have looked at porting it across:  For 2.8.3 I got it compiling cleanly on a RaspPi but the AMD specifics were a drag.  I have tried again with 2.14.1 but there is a large change to the number of parameters that various functions take.  I fixed the number of parameters (but not the internals) but ran into linker issues a couple of nights back.