PR25230, addr2line fails on dwz output

PR25230, addr2line fails on dwz output

This patch remedies the following DW_FORM_GNU_ref_alt related problem:
      /* FIXME: Do we need to locate the correct CU, in a similar
        fashion to the code in the DW_FORM_ref_addr case above ?  */
Without the correct CU the wrong abbrevs are used, resulting in
errors and/or wrong file names.

There is scope for further work here.  Parsing of CUs should be a two
step process, with the first stage just finding the bounds of the CU.
This would allow find_abstract_instance to quickly find the CU
referenced by DW_FORM_ref_addr or DW_FORM_GNU_ref_alt, then take the
second stage of CU parsing where abbrevs, ranges and suchlike consume
time and memory.  As it is, we just process CUs from the start of
.debug_info until we find the one of interest.  The testcase in the PR
takes 98G of virtual memory.

	PR 25230
	* dwarf2.c (struct dwarf2_debug_file): Add line_table and
	abbrev_offsets.
	(struct abbrev_offset_entry): New.
	(hash_abbrev, eq_abbrev, del_abbrev): New functions.
	(read_abbrevs): Check whether we have already read abbrevs at
	given offset, and add new offset/abbrev to hash table.
	(decode_line_info): Keep line table at offset zero in file struct.
	Return this for a cu reusing the same dir/file list.
	(find_abstract_instance): Find cu for DW_FORM_GNU_ref_alt.
	(_bfd_dwarf2_slurp_debug_info): Create offset/abbrev hash tables.
	(_bfd_dwarf2_cleanup_debug_info): Adjust deletion of lines and
	abbrevs.