Properly set sh_info for .rela.plt/rel.plt section

Properly set sh_info for .rela.plt/rel.plt section

Since .rela.plt/rel.plt section may contain relocations against .got.plt
section, we set sh_info for .rela.plt/rel.plt section to .got.plt section
index if target has .got.plt section.

bfd/

	PR ld/18169
	* elf-bfd.h (elf_backend_data): Add get_reloc_section.
	(_bfd_elf_get_reloc_section): New.
	* elf.c (_bfd_elf_get_reloc_section): Likewise.
	(assign_section_numbers): Call get_reloc_section to look up the
	section the relocs apply.
	* elfxx-target.h (elf_backend_get_reloc_section): Likewise.
	(elfNN_bed): Initialize get_reloc_section with
	elf_backend_get_reloc_section.

ld/testsuite/

	PR ld/18169
	* ld-elf/linkinfo1a.d: Updated.
	* ld-elf/linkinfo1b.d: Likewise.