kbuild, bpf: Enable --btf_features=attributes

pahole v1.30 has a BTF encoding feature for arbitrary attributes, used
in particular for tagging bpf_arena_alloc_pages and
bpf_arena_free_pages BPF kfuncs [1][2].

Enable it for the kernel build.

[1] https://lore.kernel.org/bpf/20250130201239.1429648-1-ihor.solodrai@linux.dev/
[2] https://lore.kernel.org/bpf/20250228194654.1022535-1-ihor.solodrai@linux.dev/



Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20250414185918.538195-1-ihor.solodrai@linux.dev