llvmpipe: Don't wait for already-terminated threads on Windows

llvmpipe: Don't wait for already-terminated threads on Windows

In the case of an app returning from main(), Windows will apparently terminate
other threads before invoking final cleanup on the main thread.

llvmpipe can't wait for threads to signal a semaphore if the thread is already
gone. Since we're already in a WIN32 special case, just call the Win32 API
to check if the thread is terminated or STILL_ALIVE.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12731>