Pass __KERNEL_EXPERIMENTAL__ to OpenCL split kernels
Pass __KERNEL_EXPERIMENTAL__ to OpenCL split kernels

Experimental feature set id currently unavailable for megakernel, it'll
require some changes to the cache system to distinguish cached regular
kernels from cached experimental kernels.

Currently unused, but some features will be enabled soon.

tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird