kernel.mk: Refactor LINUX_KARCH affectation

Switch to a dumber implementation that will be easier to maintain in the long
run, with only if statements instead of having nested subst calls.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43671 3c298f89-4303-0410-b956-a3cf2f4a3e73
kernel.mk: Handle the x86_64 LINUX_KARCH case

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