tsan: deflake dlopen_static_tls.cpp

[dvyukov]

Summary

Currently the test calls dlclose in the thread
concurrently with the main thread calling a function
from the dynamic library. This is not good.
Wait for the main thread to call the function
before calling dlclose.

Depends on D115612.

compiler-rt/test/tsan/Linux/dlopen_static_tls.cpp

 void *lib;
 void (*func)();
 int ready;
 
 void *thread(void *arg) {
   barrier_wait(&barrier);
   if (__atomic_load_n(&ready, __ATOMIC_ACQUIRE))
     func();
+  barrier_wait(&barrier);
   if (dlclose(lib)) {
     printf("error in dlclose: %s\n", dlerror());
     exit(1);
   }
   return 0;
 }
 
 int main(int argc, char *argv[]) {
   barrier_init(&barrier, 2);
   pthread_t th;
   pthread_create(&th, 0, thread, 0);
   lib = dlopen((std::string(argv[0]) + "-so.so").c_str(), RTLD_NOW);
   if (lib == 0) {
     printf("error in dlopen: %s\n", dlerror());
     return 1;
   }
   func = (void (*)())dlsym(lib, "sofunc");
   if (func == 0) {
     printf("error in dlsym: %s\n", dlerror());
     return 1;
   }
   __atomic_store_n(&ready, 1, __ATOMIC_RELEASE);
   barrier_wait(&barrier);
   func();
+  barrier_wait(&barrier);
   pthread_join(th, 0);
   fprintf(stderr, "DONE\n");
   return 0;
 }
 
 #endif // BUILD_SO
 
 // CHECK: DONE