bpf: add bpf_features enum

This commit adds a kernel side enum for use in conjucntion with BTF
CO-RE bpf_core_enum_value_exists. The goal of the enum is to assist
with available BPF features detection. Intended usage looks as
follows:

  if (bpf_core_enum_value_exists(enum bpf_features, BPF_FEAT_<f>))
     ... use feature f ...

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250625182414.30659-2-eddyz87@gmail.com


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