binutils/dwarf.c: abbrev caching

I'm inclined to think that abbrev caching is counter-productive.  The
time taken to search the list of abbrevs converted to internal form is
non-zero, and it's easy to decode the raw abbrevs.  It's especially
silly to cache empty lists of decoded abbrevs (happens with zero
padding in .debug_abbrev), or abbrevs as they are displayed when there
is no further use of those abbrevs.  This patch stops caching in those
cases.

	* dwarf.c (record_abbrev_list_for_cu): Add free_list param.
	Put abbrevs on abbrev_lists here.
	(new_abbrev_list): Delete function.
	(process_abbrev_set): Return newly allocated list.  Move
	abbrev base, offset and size checking to..
	(find_and_process_abbrev_set): ..here, new function.  Handle
	lookup of cached abbrevs here, and calculate start and end
	for process_abbrev_set.  Return free_list if newly alloc'd.
	(process_debug_info): Consolidate cached list lookup, new list
	alloc and processing into find_and_process_abbrev_set call.
	Free list when not cached.
	(display_debug_abbrev): Similarly.