tsan: add a test for on_exit

[dvyukov]

Summary

Depends on D114605.

compiler-rt/test/tsan/on_exit.cpp

+// RUN: %clang_tsan -O1 %s -o %t && %deflake %run %t | FileCheck %s
+#include "test.h"
+
+volatile long global;
+
+void *thread(void *x) {
+  global++;
+  barrier_wait(&barrier);
+  return nullptr;
+}
+
+void on_exit_callback(int status, void *arg) {
+  fprintf(stderr, "on_exit_callback(%d, %lu)\n", status, (long)arg);
+  global++;
+}
+
+int main() {
+  on_exit(on_exit_callback, (void *)42l);
+  barrier_init(&barrier, 2);
+  pthread_t th;
+  pthread_create(&th, nullptr, thread, nullptr);
+  pthread_detach(th);
+  barrier_wait(&barrier);
+  return 2;
+}
+
+// CHECK: on_exit_callback(2, 42)
+// CHECK: WARNING: ThreadSanitizer: data race
+// CHECK:   Write of size 8
+// CHECK:     #0 on_exit_callback
+// CHECK:     #1 on_exit_wrapper