libbpf: Split feature detectors definitions from cached results

Split a list of supported feature detectors with their corresponding
callbacks from actual cached supported/missing values. This will allow
to have more flexible per-token or per-object feature detectors in
subsequent refactorings.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20240124022127.2379740-22-andrii@kernel.org