armv8: Fix defconfig build

Commit 14f48861 ("arc: Add ARC and ARC BE architecture") introduced a
sed match against arc* buildroot architecture to translate it to arc.

This causes a problem with armv8, which is called in buildroot aarch64,
which is also matching, resulting in a build failure.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>