x64 is handled by the x86 architecture in Linux, add a case for it in LINUX_KARCH. Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com> git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43672 3c298f89-4303-0410-b956-a3cf2f4a3e73