PR29370, infinite loop in display_debug_abbrev

The PR29370 testcase is a fuzzed object file with multiple
.trace_abbrev sections.  Multiple .trace_abbrev or .debug_abbrev
sections are not a violation of the DWARF standard.  The DWARF5
standard even gives an example of multiple .debug_abbrev sections
contained in groups.  Caching and lookup of processed abbrevs thus
needs to be done by section and offset rather than base and offset.
(Why base anyway?)  Or, since section contents are kept, by a pointer
into the contents.

	PR 29370
	* dwarf.c (struct abbrev_list): Replace abbrev_base and
	abbrev_offset with raw field.
	(find_abbrev_list_by_abbrev_offset): Delete.
	(find_abbrev_list_by_raw_abbrev): New function.
	(process_abbrev_set): Set list->raw and list->next.
	(find_and_process_abbrev_set): Replace abbrev list lookup with
	new function.  Don't set list abbrev_base, abbrev_offset or next.