libbpf: integrate __arg_ctx feature detector into kernel_supports()

Now that feature detection code is in bpf-next tree, integrate __arg_ctx
kernel-side support into kernel_supports() framework.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240125205510.3642094-2-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>