Fix q8_0 test in test-quantize-fns

vec_signed uses unexpected rounding mode.
Explicitly use different rounding function.

Author: AlekseiNikiforovIBM

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

@@ -75,7 +75,8 @@ void quantize_row_q8_0(const float * GGML_RESTRICT x, void * GGML_RESTRICT vy, i
 
         for (int j = 0; j < 8; j++) {
             const float32x4_t v = vec_mul(srcv[j], vec_splats(id));
-            const int32x4_t vi = vec_signed(v);
+            /* Uses non-default rounding for vec_signed or vec_round */
+            const int32x4_t vi = vec_signed(__builtin_s390_vfisb(v, 0, 1));
 
             y[i].qs[4*j + 0] = vec_extract(vi, 0);
             y[i].qs[4*j + 1] = vec_extract(vi, 1);
@@ -122,7 +123,8 @@ void quantize_row_q8_1(const float * GGML_RESTRICT x, void * GGML_RESTRICT vy, i
 
         for (int j = 0; j < 8; j++) {
             const float32x4_t v = vec_mul(srcv[j], vec_splats(id));
-            const int32x4_t vi = vec_signed(v);
+            /* Uses non-default rounding for vec_signed or vec_round */
+            const int32x4_t vi = vec_signed(__builtin_s390_vfisb(v, 0, 1));
 
             y[i].qs[4*j + 0] = vec_extract(vi, 0);
             y[i].qs[4*j + 1] = vec_extract(vi, 1);