selftests/bpf: Fix kprobe_multi_test/attach_override test

We need to deny the attach_override test for arm64, denying the
whole kprobe_multi_test suite. Also making attach_override static.

Fixes: 7182e564 ("selftests/bpf: Add kprobe_multi override test")
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230913114711.499829-1-jolsa@kernel.org