You have /usr/lib in the linker search-path, that's normally wrong for cross-compiling, because it contains system-libs.
You might ned to explicitely set the path where the libm.so for your microcontroller is to the linker search paths.
dpkg -L <package-name> lists all files belonging to the package.
In your case dpkg -L avr-libc.
If it is not installed, you can install it with sudo apt-get install avr-libc.