SignalDelegator: Migrate from std::unexpected() to std::terminate()

std::unexpected() was deprecated in C++11 and removed from the standard
in C++17. The default unexpected_handler would call std::terminate
anyway, and given we don't explicitly set a termination handler (as far
as I can tell), this retains identical behavior.

Source/Tests/LinuxSyscalls/SignalDelegator.cpp

@@ -170,7 +170,7 @@ namespace FEX::HLE {
 
           // Doesn't return
           FEXCore::Context::StopThread(Thread->CTX, Thread);
-          std::unexpected();
+          std::terminate();
         }
       }
       else if (Handler.GuestAction.sigaction_handler.handler == SIG_IGN) {