Select a default kernel optimized for big cores.
The A57 kernel is the A75 kernel with prefetch removed.

PiperOrigin-RevId: 285865346
diff --git a/src/init.c b/src/init.c
index 98ffe80..c5ff109 100644
--- a/src/init.c
+++ b/src/init.c
@@ -441,11 +441,11 @@
           break;
         default:
           xnn_params.f32.gemm = (struct gemm_parameters) {
-            .gemm = (xnn_gemm_ukernel_function) xnn_f32_gemm_ukernel_6x8__neonfma_lane_ld64,
-            .igemm = (xnn_igemm_ukernel_function) xnn_f32_igemm_ukernel_6x8__neonfma_lane_ld64,
+            .gemm = (xnn_gemm_ukernel_function) xnn_f32_gemm_ukernel_4x8__aarch64_neonfma_cortex_a57,
+            .igemm = (xnn_igemm_ukernel_function) xnn_f32_igemm_ukernel_4x8__aarch64_neonfma_cortex_a75,
             .gemm1 = (xnn_gemm_ukernel_function) xnn_f32_gemm_ukernel_1x8__aarch64_neonfma_cortex_a75,
             .igemm1 = (xnn_igemm_ukernel_function) xnn_f32_igemm_ukernel_1x8__aarch64_neonfma_cortex_a75,
-            .mr = 6,
+            .mr = 4,
             .nr = 8,
           };
           break;