Clamp NC operator for S8 data type

- New API functions: xnn_create_clamp_nc_s8 and xnn_setup_clamp_nc_s8
- Unit tests

PiperOrigin-RevId: 391216240
diff --git a/src/operator-strings.c b/src/operator-strings.c
index fcab61d..30e8fe9 100644
--- a/src/operator-strings.c
+++ b/src/operator-strings.c
@@ -44,6 +44,8 @@
       return "Channel Shuffle (NC, X32)";
     case xnn_operator_type_clamp_nc_f32:
       return "Clamp (NC, F32)";
+    case xnn_operator_type_clamp_nc_s8:
+      return "Clamp (NC, S8)";
     case xnn_operator_type_clamp_nc_u8:
       return "Clamp (NC, U8)";
     case xnn_operator_type_constant_pad_nd_x8: