Assert size of elf_x86_64_{bnd|legacy}_plt2_entry

Assert size of elf_x86_64_{bnd|legacy}_plt2_entry

Assert size of elf_x86_64_bnd_plt2_entry and elf_x86_64_legacy_plt2_entry
only in elf_x86_64_check_relocs.

	* elf64-x86-64.c (elf_x86_64_check_relocs): Assert size of
	elf_x86_64_bnd_plt2_entry and elf_x86_64_legacy_plt2_entry.
	(elf_x86_64_allocate_dynrelocs): Don't assert size of
	elf_x86_64_bnd_plt2_entry and elf_x86_64_legacy_plt2_entry.