dwarf_loader: Print the CU's language when a tag isn't supported

To help debug --lang_exclude, for instance, now, even with
--lang_exclude=rust, we're getting this when processing a recent Fedora
40 kernel:

  $ uname -a
  Linux toolbox 6.11.3-200.fc40.x86_64 #1 SMP PREEMPT_DYNAMIC Thu Oct 10 22:31:19 UTC 2024 x86_64 GNU/Linux
  $ pahole --running_kernel_vmlinux
  /home/acme/.cache/debuginfod_client/8c97f87b685044733f5d1576eca82df1f3b7fa0c/debuginfo
  acme@x1:~/git/pahole$ tests/btf_functions.sh
  Validation of BTF encoding of functions; this may take some time: die__process_class: tag not supported 0x33 (variant_part) at <7719ef8>!
  die__create_new_enumeration: DW_TAG_subprogram (0x2e) @ <0x77370f1> not handled in a rust CU!
  tag__recode_dwarf_type: couldn't find name for function 0x77558a9, abstract_origin=0, specification=0x77370f1
  ^C
  $

Cc: Alan Maguire <alan.maguire@oracle.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Stephen Brennan <stephen.s.brennan@oracle.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>