Add nocluster.c file with empty synch_barrier

This makes many tests compatible with execution on PULPissimo

kernel/nocluster.c

@@ -0,0 +1,24 @@
+/*
+ * Copyright (C) 2025 ETH Zurich and University of Bologna
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "pulp.h"
+#include <stdio.h>
+#include <stdlib.h>
+
+void synch_barrier()
+{
+  return;
+}

rules/pulpos/src.mk

@@ -13,4 +13,6 @@ PULP_ASM_SRCS += kernel/irq_asm.S
 
 ifneq '$(cluster/version)' ''
 PULP_SRCS += kernel/cluster.c
+else
+PULP_SRCS += kernel/nocluster.c
 endif