- specifically on Windows: the `sentry_thread_spawn` macro is defined as follows:

```
#    define sentry__thread_spawn(ThreadId, Func, Data)                         \
        (*ThreadId = CreateThread(NULL, 0, Func, Data, 0, NULL),               \
            *ThreadId == INVALID_HANDLE_VALUE ? 1 : 0)
```

We can see that `ThreadId` is initialized with the value returned by
`CreateThread`, but the new thread might be running before `CreateThread`
returns.

In this change a new function `sentry__thread_get_current_threadid` is
added, which returns the current thread id in a reliable way.