Issue was caused by how CUDA devices availability done in Cycles.
Basically, if there's no WITH_CUDA_BINARIES buildtime, nvcc becomes
mandatory dependency.
Since kernels are building in separate target now, this logic broke
a bit.
Perhaps condition in util_cuda shall be changed to be a bit smarter,
but for now just work-around by enabling CUDA binaries when building
Cycles. Made it empty arch list to be sure no kernels will try to
re-compile after cudakernels target is done.
dc4636827b