gold: Get linkonce/comdate sections for debugging sections

gold: Get linkonce/comdate sections for debugging sections

When relocating debug sections, get the section index for the linkonce
section.  Since symbols referenced in debugging sections can be defined
a single comdat section with a different section name, also check the
single comdat section.

	PR gold/26937
	* object.cc (Sized_relobj_file::map_to_kept_section): Get the
	section index for linkonce section.  Also check the single
	comdat section.
	* testsuite/Makefile.am (check_SCRIPTS): Add pr26936.sh.
	(check_DATA): Add pr26936a.stdout and pr26936b.stdout.
	(MOSTLYCLEANFILES): Add pr26936a and pr26936b.
	(pr26936a.stdout): New target.
	(pr26936a): Likewise.
	(pr26936b.stdout): Likewise.
	(pr26936b): Likewise.
	(pr26936a.o): Likewise.
	(pr26936b.o): Likewise.
	(pr26936c.o): Likewise.
	(pr26936d.o): Likewise.
	* testsuite/Makefile.in: Regenerated.
	* testsuite/pr26936.sh: New file.
	* testsuite/pr26936a.s: Likewise.
	* testsuite/pr26936b.s: Likewise.
	* testsuite/pr26936c.s: Likewise.
	* testsuite/pr26936d.s: Likewise.