Improve opencl kernel caching

[vmx]

There is a report about problems with cached versions of the opencl kernel at filecoin-project/community#172 (comment):

So every time you change drivers or OpenCL libraries or lotus versions (when lotus has updated extern/filecoin-ffi), it can help to avoid problems by clearing the kernel cache and letting lotus re-generate it.

Ideally those changes are recognized by the caching mechanism and they get re-created. Detecting everything e.g. driver upgrades might not be possible though. But at least checking for the FFI version should be.