libsigsegv: fix avr32 build

The libsigsegv configure step fails to correctly figure out the direction of
stack growth for avr32. This leads to a zero STACK_DIRECTION definition, and
build failures such as the following.

  http://autobuild.buildroot.net/results/a3fe938f9376533b4777d79deb7a2ee83ed5ce33



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