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>

diff -Nurp a/m4/stack-direction.m4 b/m4/stack-direction.m4
--- a/m4/stack-direction.m4	2009-11-21 12:22:45.000000000 +0000
+++ b/m4/stack-direction.m4	2013-11-04 09:29:27.558036038 +0000
@@ -16,7 +16,7 @@ AC_DEFUN([SV_STACK_DIRECTION],
       alpha* | \
       arc | \
       arm* | strongarm* | xscale* | \
-      avr | \
+      avr | avr32 | \
       c1 | c2 | c32 | c34 | c38 | \
       clipper | \
       cris | \