PR25333, GAS is slow processing -fdebug-types-sections

gas needs to build lists of sections for each group.  This arranges to
build the lists earlier, so they can be used when looking for sections
that belong to a group.  Using the section hash table to find sections
by name, then by group isn't efficient when there are numerous groups
with the same section names.  Using a hash table to quickly find a
group, then searching by section name on a list for the group results
in a 100-fold speed improvement assembling the testcase in this PR.

To reduce the number of times we traverse the section list, the patch
also moves some processing done in elf_adjust_symtab for linked-to
section, to elf_frob_file.  This requires a testsuite change because
processing will stop before elf_frob_file if there is a parse error in
section21.s, ie. you'll only get the "junk at end of line" error, not
the "undefined linked-to symbol" errors.

	PR 25333
	* config/obj-elf.c (struct group_list, groups): Move earlier.
	(match_section): New function, extracted from..
	(get_section_by_match): ..here.
	(free_section_idx): Move earlier.
	(group_section_find, group_section_insert): New functions.
	(change_section): Use the above.
	(elf_set_group_name): New function.
	(obj_elf_attach_to_group): Use elf_set_group_name.
	(set_additional_section_info): Handle linked_to_symbol_name and
	stabs code, extracted from..
	(adjust_stab_sections): ..here,..
	(build_additional_section_info): ..and here.
	(elf_adjust_symtab): Don't call build_additional_section_info.
	(elf_frob_file): Adjust.
	* config/obj-elf.h (elf_set_group_name): Declare.
	* config/tc-xtensa.c (cache_literal_section): Use elf_set_group_name.
	(xtensa_make_property_section): Likewise.
	* testsuite/gas/elf/attach-1.d: Stricter group section matching,
	and changed group section ordering.
	* testsuite/gas/elf/attach-2.d: Stricter group section matching.
	* testsuite/gas/elf/attach-2.s: Provide section bar type.
	* testsuite/gas/elf/elf.exp: Run attach-2.
	* testsuite/gas/elf/section21.l: Update.
	* testsuite/gas/elf/section21.s: Don't check for a parse error.