I would say that Cilk is more comparable to C++ AMP (or, to a lesser extent OpenCL) -- a "parallel programming language" (not a great term but I can't think of a better one at the moment) which can potentially be implemented on a number of different hardware platforms (although I don't know if there are plans to port Cilk to other platforms).
HSA is more of a platform specification designed to optimally support parallel programming languages across multiple hardware and OS platforms.
HSA Foundation ARM, AMD, Imagination, MediaTek, Qualcomm, Samsung, TI
AMD Announces Heterogeneous C++ AMP Language for Developers - HSA Foundation