gendwarfksyms: Expand subroutine_type

gendwarfksyms: Expand subroutine_type

Add support for expanding DW_TAG_subroutine_type and the parameters
in DW_TAG_formal_parameter. Use this to also expand subprograms.

Example output with --dump-dies:

  subprogram (
    formal_parameter pointer_type {
      const_type {
        base_type char byte_size(1) encoding(6)
      }
    }
  )
  -> base_type unsigned long byte_size(8) encoding(7)

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>