Fix q3_K dot product error in test-quantize-fns on s390x

Array q8bytes had only 4 elements allocated, but 8 elements accessed.
This lead to write out of bounds and later read of overwritten values out of bounds
and incorrect result.

Author: AlekseiNikiforovIBM

ggml/src/ggml-cpu/arch/s390/quants.c

@@ -638,7 +638,7 @@ void ggml_vec_dot_q3_K_q8_K(int n, float * GGML_RESTRICT s, size_t bs, const voi
     uint8x16_t q3h[4];
     uint8x16_t q3b[2];
     int8x16_t q3bytes[4];
-    int8x16_t q8bytes[4];
+    int8x16_t q8bytes[8];
     uint8x16_t qhbits[2];
 
     float sum = 0;