add x86_64 target support

Signed-off-by: Imre Kaloz <kaloz@openwrt.org>



git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39208 3c298f89-4303-0410-b956-a3cf2f4a3e73
Merge branch 'master' of git://git.openwrt.org/openwrt into add-gccgo-and-libgo