Count R_X86_64_RELATIVE64 as reloc_class_relative

bfd/

	* elf64-x86-64.c (elf_x86_64_reloc_type_class): Handle
	R_X86_64_RELATIVE64.

ld/testsuite/

	* ld-x86-64/pr13082-1a.d: Check RELACOUNT.
	* ld-x86-64/pr13082-1b.d: Likewise.