PR26265, Spurious 'gc-sections requires ... when --init or --fini

PR26265, Spurious 'gc-sections requires ... when --init or --fini

bfd/
	* elflink.c (_bfd_elf_gc_keep): Use bfd_is_const_section.
ld/
	PR 26265
	* ldlang.c (undef_from_cmdline): Delete.
	(ldlang_add_undef): Mark "cmdline" param unused.
	(lang_end): Traverse gc_sym_list to determine whether a symbol root
	has been specified.  Update error message.
	* testsuite/ld-gc/noent.d: Adjust for changed error message.