[mono-threads] Move platform unregister after detach (#3604)

[mono-threads] Move platform unregister after detach (#3604)

This is important as platform unregister is where we signal the thread handle, and that should be done as late as possible, but still before we switch the thread state to DETACHED and remove it from the thread list.

The fact that we do under the suspend lock shouldn't be an issue as we shouldn't call in any other code than w32handle code.