Ngdbuild issues a warning on the instantiated READBACK component in your HDL:
warning basnu:93-logical block 'rebk1' of type 'READBACK' is unexpanded.
Exemplar's Galileo 4.2.2 incorrectly declares the READBACK symbol in the xc4000 libraries. The READBACK component is actually composed of 2 components: 1 RDBK and 1 RDCLK. If these 2 components are declared instead of READBACK, the design will be able to translate through ngdbuild. The work around is:
Note: Instantiating the RDCLK indicates to the FPGA that the USER clock will be used for readback. If this is not the case, then only instantion of the RDBK is necessary, and the FPGA will use the onboard CCLK for readback.