adjust guarantee in Handshake::execute for the case of target thread being current

The following guarantee in Handshake::execute has to be adjusted for the case of target thread being current:
void Handshake::execute(HandshakeClosure* hs_cl, ThreadsListHandle* tlh, JavaThread* target) {
  . . .
  if (tlh == nullptr) {
    guarantee(Thread::is_JavaThread_protected_by_TLH(target),
              "missing ThreadsListHandle in calling context.");

It has to be similar to the following async Handshake::execute version:

void Handshake::execute(AsyncHandshakeClosure* hs_cl, JavaThread* target) {
  jlong start_time_ns = os::javaTimeNanos();
  AsyncHandshakeOperation* op = new AsyncHandshakeOperation(hs_cl, target, start_time_ns);

  guarantee(target != nullptr, "must be");

  Thread* current = Thread::current();
  if (current != target) {
    // Another thread is handling the request and it must be protecting
    // the target.
    guarantee(Thread::is_JavaThread_protected_by_TLH(target),
              "missing ThreadsListHandle in calling context.");
  }

Also, please, refer to the comments from David on the related bug JDK-8286960