PR 21412, get_reloc_section assumes .rel/.rela name for SHT_REL/RELA.

PR 21412, get_reloc_section assumes .rel/.rela name for SHT_REL/RELA.

This patch fixes an assumption made by code that runs for objcopy and
strip, that SHT_REL/SHR_RELA sections are always named starting with a
.rel/.rela prefix.  I'm also modifying the interface for
elf_backend_get_reloc_section, so any backend function just needs to
handle name mapping.

	PR 21412
	* elf-bfd.h (struct elf_backend_data <get_reloc_section>): Change
	parameters and comment.
	(_bfd_elf_get_reloc_section): Delete.
	(_bfd_elf_plt_get_reloc_section): Declare.
	* elf.c (_bfd_elf_plt_get_reloc_section, elf_get_reloc_section):
	New functions.  Don't blindly skip over assumed .rel/.rela prefix.
	Extracted from..
	(_bfd_elf_get_reloc_section): ..here.  Delete.
	(assign_section_numbers): Call elf_get_reloc_section.
	* elf64-ppc.c (elf_backend_get_reloc_section): Define.
	* elfxx-target.h (elf_backend_get_reloc_section): Update.