Extract atomic-gcc-norace example from klib

tests/regression/03-practical/46-atomic-gcc-norace.c

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+// Race-free due to GCC atomic operation.
+// Extracted from concrat/klib.
+#include <stdlib.h>
+#include <pthread.h>
+#include <goblint.h>
+extern int __VERIFIER_nondet_int();
+
+int data;
+
+void *thread(void *arg) {
+  __sync_fetch_and_add(&data, 1); // NORACE
+  return NULL;
+}
+
+int main() {
+  int threads_total = __VERIFIER_nondet_int();
+  __goblint_assume(threads_total >= 0);
+
+  pthread_t *tids = malloc(threads_total * sizeof(pthread_t));
+
+  // create threads
+  for (int i = 0; i < threads_total; i++) {
+    pthread_create(&tids[i], NULL, &thread, NULL); // may fail but doesn't matter
+  }
+
+  // join threads
+  for (int i = 0; i < threads_total; i++) {
+    pthread_join(tids[i], NULL);
+  }
+
+  free(tids);
+
+  return 0;
+}