default-configs: Add config for aarch64-linux-user

Add a config for aarch64-linux-user, thereby enabling it as
a valid target.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Richard Henderson <rth@twiddle.net>
