As explained here http://www.arduino.cc/playground/Linux/Gentoo , after using crossdev to build avr-g++, it's necessary to do this before you can do builds in arduino:
ln -sv /usr/lib/binutils/avr/2.20.1/ldscripts/ /usr/avr/lib/ldscripts
Maybe crossdev could save us the trouble? Or maybe there is a better fix than that.
Steps to Reproduce:
If you have not created that symlink, arduino will complain like this:
/usr/libexec/gcc/avr/ld: cannot open linker script file ldscripts/avr5.x: No such file or directory
*** This bug has been marked as a duplicate of bug 147155 ***