Apply a workaround to mitigate a quadratic performance hit in the linker when writing...

Apply a workaround to mitigate a quadratic performance hit in the linker when writing out secondary reloc sections.

	PR 26406
	* elf-bfd.h (struct bfd_elf_section_data): Add
	has_secondary_relocs field.
	* elf.c (_bfd_elf_copy_special_section_fields): Set the
	has_secondary_relocs field for sections which have associated
	secondary relocs.
	* elfcode.h (elf_write_relocs): Only call write_secondary_relocs
	on sections which have associated secondary relocs.