Introduce Thread::as_Java_thread() convenience function

It is very common to see code like this:

void foo(Thread* thread) {
  assert(thread->is_Java_thread(), "must be");
  JavaThread* jthread = (JavaThread*) thread;

This can be simplified by adding as_Java_thread:

JavaThread* Thread::as_JavaThread() {
  assert(is_Java_thread(), "must be");
  return static_cast<JavaThread*>(this);
}

so we can then simply have:

void foo(Thread* thread) {
  JavaThread* jthread = thread->as_Java_thread();