BTF encoder:

- Add support to BTF_KIND_ENUM64 to represent enumeration entries
  with more than 32 bits.

- Support multithreaded encoding, in addition to DWARF
  multithreaded loading, speeding up the process.

  Selected just like DWARF multithreaded loading, using the
  'pahole -j' option.

- Encode 'char' type as signed.

BTF Loader:

- Add support to BTF_KIND_ENUM64.

pahole:

- Introduce --lang and --lang_exclude to specify the language the
  DWARF compile units were originated from to use or filter.

  Use case is to exclude Rust compile units while aspects of the
  DWARF generated for it get sorted out in a way that the kernel
  BPF verifier don't refuse loading the BTF generated from them.

- Introduce --compile to generate compilable code in a similar fashion to:

   bpftool btf dump file vmlinux format c > vmlinux.h

  As with 'bpftool', this will notice type shadowing, i.e. multiple types
  with the same name and will disambiguate by adding a suffix.

- Don't segfault when processing bogus files.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>