PR27259, SHF_LINK_ORDER self-link

This stops ld from endless looping on SHF_LINK_ORDER sh_link loops.

bfd/
	PR 27259
	* elflink.c (_bfd_elf_gc_mark_extra_sections): Use linker_mark to
	prevent endless looping of linked-to sections.
ld/
	PR 27259
	* ldelf.c (ldelf_before_place_orphans): Use linker_mark to
	prevent endless looping of linked-to sections.

(cherry picked from commit def97fb9)