dwarf-reader: Fix DWARF string comparison optimization

dwarf-reader: Fix DWARF string comparison optimization

This fix has been triggered by a fix posted by Thomas Schwinge in the
thread that started at https://sourceware.org/pipermail/libabigail/2022q1/004139.html.

Thomas rightfully notes that compare_dies_string_attribute_value is
wrong.  In some cases, it wrongly considers only the first character
of the two strings to compare.

This patch fixes that and updates the regression tests accordingly.
The fix suppresses several spurious changes (in runtestdifffilter)
that were there and I never got the bottom of them.  Now they are
gone.

	* src/abg-dwarf-reader.cc (slowly_compare_strings)
	(die_char_str_attribute): Define new static functions.
	(compare_dies_string_attribute_value): Use the new
	slowly_compare_strings here.
	* tests/data/test-annotate/test15-pr18892.so.abi: Adjust.
	* tests/data/test-annotate/test17-pr19027.so.abi: Likewise.
	* tests/data/test-annotate/test19-pr19023-libtcmalloc_and_profiler.so.abi: Likewise.
	* tests/data/test-read-dwarf/PR25042-libgdbm-clang-dwarf5.so.6.0.0.abi: Likewise.
	* tests/data/test-read-dwarf/test-libandroid.so.abi: Likewise.
	* tests/data/test-read-dwarf/test15-pr18892.so.abi: Likewise.
	* tests/data/test-read-dwarf/test17-pr19027.so.abi: Likewise.
	* tests/data/test-read-dwarf/test19-pr19023-libtcmalloc_and_profiler.so.abi: Likewise.
	* tests/data/test-read-dwarf/test22-pr19097-libstdc++.so.6.0.17.so.abi: Likewise.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>