changes-v1.31

BTF encoder:

- Rework the selection of functions to represent in BTF, for instance:

  - Skip functions that passes values thru the stack when those structs don't
    have expected alignment due to some attribute usage that then causes
    problems with BTF trampolines due to lack of expressiveness in BTF to
    signal such special cases.

- Skip objects (compile units) without DWARF: don't stop a multi object
  encoding session just because one doesn't have any DWARF in it.

- Fix BTF dedup by updating libbpf. 

BTF loader:

- Fix the inference of the explicit alignment attribute of zero length arrays,
  like struct skb_ext->data[] in the Linux kernel. Important as BTF has no no
  explicit alignment attribute encoding.

- Fix the inference of alignments after bitfields, such as in struct
  nft_rule_dp->data[] after ->handle:42, also in the Linux kernel.
    
pahole:

- Fix segfault with --show_reorg_steps option, e.g. pahole -R -S -C task_struct.

CI:

- Add comparision of functions encoded in BTF between baseline 'master' branch
  and current branch, i.e. 'next'.