eina: Implement Eina_Thread for native windows
The implementation design respects the fact that Eina_Thread is an
uintptr_t. Thus we allocate the thread struct in the heap and return a
pointer to it.
As such, we store the created thread structure in the target thread
TLS slot. For threads that were not created through eina API, in
eina_thread_self we allocate a new structure, push it to the TLS slot
and mark it to be freed on thread exit.