[vdso] extract vdso function from linux-vdso.so.1

Extract vdso function from DSO linux-vdso.so.1
We use vdso version of getcpu(...) for example; and its signature is

__vdso_getcpu

using Func =  int (*)(unsigned* cpu, unsigned* node, void* unused);

Func resolveVdsoFunc() {
#if !defined(FOLLY_HAVE_LINUX_VDSO) || defined(FOLLY_SANITIZE_MEMORY)
  return nullptr;
#else
  void* h = dlopen("linux-vdso.so.1", RTLD_LAZY | RTLD_LOCAL | RTLD_NOLOAD);
  if (h == nullptr) {
    return nullptr;
  }

  auto func = Func(dlsym(h, "__vdso_getcpu"));
  if (func == nullptr) {
    // technically a null result could either be a failure or a successful
    // lookup of a symbol with the null value, but the second can't actually
    // happen for this symbol.  No point holding the handle forever if
    // we don't need the code
    dlclose(h);
  }

  return func;
#endif
}

We could find this usage in Folly lib: https://github.com/facebook/folly/blob/535d0313371b1c8d313ec8a1cd09b940fa992397/folly/concurrency/CacheLocality.cpp#L310