util-linux: disable fallocate for avr32

The fallocate syscall is not implemented in the avr32 toolchain.

Fixes build failures such as the following.

  http://autobuild.buildroot.net/results/bc4/bc41a3fea20181526eb247ac910244aa2aa4c4c0/



Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>