Referring to #408, this commit #ifdefs DSPr2 SIMD functions that only work on little endian processors, and it completely excludes jsimd_h2v1_downsample_dspr2() and jsimd_h2v2_downsample_dspr2(). The latter two functions fail with the TJBench tiling regression tests, most likely because the implementation of the functions predates those tests.