For mingw builds the objects file are located in an mri script as input for ar.
The location of the mri sript depends on the output target. This information is missing in the usage of the mri script.

So perhaps the mingw_make.cpp(376) could be changed to
objectsLinkLine = "ar -M < " + Option::output_dir + QDir::separator() + ar_script_file;
to use in shadow builds

Workaround:
As far as i know i can inrease QMAKE_LINK_OBJECT_MAX, but this will lead in feature to other problems (cmd line length etc)