[DevTools] Fix ReactDevToolsBackend module for AMD (#35891)

## Summary

For apps that use AMD, we need to actually `require()` the
ReactDevToolsBackend and load it from the AMD module cache. This adds a
check for the case where the `ReactDevToolsBackend` isn't defined
globally, and so we load it with `require()`.


## How did you test this change?

Tested through https://github.com/facebook/react/pull/35886