Both Xilinx and Altera make the assumption that their tools are being run on a ridiculously obsolete versions of Linux. The Quartus tools test to see if they are running on Redhat 8, if not they set the LD_ASSUME_KERNEL to 2.4. At one point the Xilinx tools had a requirement that you set the LD_ASSUME_KERNEL to 2.4.7. This didn't cause any problems in earlier version of Fedora Core but it does in FC5. If you set the LD_ASSUME_KERNEL to 2.4 it breaks everything, see below
setenv LD_ASSUME_KERNEL 2.4 ls ls: error while loading shared libraries: librt.so.1: wrong ELF class: ELFCLASS32
The good news is that it appears that it's no longer necessary to set the LD_ASSUME_KERNEL for either tool. Both ISE 8.1sp3 and Quartus 6.0 seem to work without it.
Xilinx assumed that the user was setting the LD_ASSUME_KERNEL variable so those of you who use scripts or set it in their .cshrc should remove it.
Altera put it in one of their own scripts, $QUARTUS/adb/qenv.csh. In order to run Quartus on FC5 you should comment out the line,
if ( "$REDHAT_VERSION" != "8" ) then # setenv LD_ASSUME_KERNEL 2.4 endif