gcc: fix avr32 default GCC version

Fix gcc version default after r36355 removing all GCC_DEFAULT_VERSION
symbols.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36688 3c298f89-4303-0410-b956-a3cf2f4a3e73