build: enable MIPS16 again, but make it depend on !gcc-4.6

gcc 4.6 miscompiles hostapd with mips16, on gcc 4.8 it works

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@37932 3c298f89-4303-0410-b956-a3cf2f4a3e73