Run() registers KytyClose with atexit(), but Execute() ends the process with
std::quick_exit(), which invokes at_quick_exit handlers only. Nothing registered
one, so the cleanup never ran and neither did the emergency shutdown handlers
that close open guest files and flush the log.
Register a quick-exit handler that runs the emergency shutdown. It deliberately
skips RuntimeLinker::Clear(): guest threads are still live at that point and
tearing the linker down there deadlocks against a thread parked in a condition
wait.
