Use "Fixed-size Sparse Matrix x Dense Matrix" (FsSpMDM) interface for `xsmm` backend

For certain applications using a non-tensor basis, the basis is quite sparse and the basis application can take advantage of this to increase performance. This FsSpMDM implementation in LIBXSMM benchmarks the sparse vs. dense kernels during construction (after JiT) and chooses the faster one. This could be an easy thing to try out in libCEED.