CC: Pass the C++ mode flag to Driver in both divc++ and diosc++.

divine/cc/native.cpp

@@ -69,6 +69,8 @@ namespace divine::cc
     {
         if ( _ld_args.empty() )
             _ld_args = cc::ld_args( _po, _files );
+        if ( _cxx )
+            _ld_args.push_back( "--driver-mode=g++" );
     }
 
     void Native::link()

divine/rt/dios-cc.cpp

@@ -76,7 +76,6 @@ auto NativeDiosCC::link_dios_native( bool cxx )
 
     if ( cxx )
     {
-        _ld_args.push_back( "--driver-mode=g++" );
         _ld_args.push_back( "-stdlib=libc++" );
         _ld_args.push_back( "-L" + tmpdir.path );
         writeFile( cxxlib, rt::libcxx() );