PR25042 - Support string form DW_FORM_strx{1,4} from DWARF 5

PR25042 - Support string form DW_FORM_strx{1,4} from DWARF 5

	* configure.ac: Detect the presence of the DW_FORM_strx{1,4}
	enumerators.
	* src/abg-dwarf-reader.cc (form_is_DW_FORM_strx): Define new
	function.
	(compare_dies_string_attribute_value): Use the new
	form_is_DW_FORM_strx here.
	* tests/data/Makefile.am: Add the new test input files below to
	source distribution.
	* tests/data/test-read-dwarf/PR25042-libgdbm-clang-dwarf5.so.6.0.0:
	New binary test input file.
	* tests/data/test-read-dwarf/PR25042-libgdbm-clang-dwarf5.so.6.0.0.abi:
	Reference output of the new binary test input file.
	* tests/test-read-dwarf.cc (in_out_specs): Add the input test
	files above to the test harness, for platforms that support the
	DW_FORM_strx form.

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