A fix for this issue is available in the 11.4 rev1 patch found below.
The patch will be included in the next release of the core.
This patch is for use with ISE 14.2 and Vivado 2012.2 design tools.
Install the patch below by extracting the contents of the ".zip" archive to the root directory of the XILINX (Xilinx ISE installation).
Select the option that allows the extractor to overwrite all of the existing files and maintain the directory structure predefined in the archive.
For further information on finding the Xilinx install and using environment variable, see (Xilinx Answer 11630).
Once the patch is installed, generate the Ethernet 1000BASE-X PCS/PMA or SGMII v11.4 core in the 14.2 CORE Generator or Vivado 2012.2 tools.
Note: You might need to have a system administrator install the patch if you do not have write permissions to the Xilinx Install directory.