Win32(core): _endthreadex(0) instead of _endthread()

Win32(core): _endthreadex(0) instead of _endthread()

According to msdn doc, _endthreadex(0) is to be used when thread is
 launched with _beginthreadex().

Note that when returning from the normal path, an implicit _endthreadex is
issued anyway.  It is important to be consistent whatever the path, since
those final functions (_endthread, ExitThread, _endthreadex) don't have
the same deallocation policy.