TARGET=CORTEXA72 previously reused the A57 8x4 path. Add a dedicated
6x8 NEON ukernel, contiguous MR=6 panel packers (stock gemm_*copy_6 is
4+2), and UNROLL_M=6-aware TRSM kernels so HPL/dtrsm does not corrupt
the heap. DTRMM falls back to generic 2x2 until a matching kernel exists.
Co-authored-by: Cursor <cursoragent@cursor.com>
