Buildbot: fix for missing CUDA compute after recent changes

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.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53748 954f8c5b-7b00-dc11-b283-0030488c597c