x86: Skip undefined symbol when finishing DT_RELR

Don't abort for undefined symbol when finishing DT_RELR.  Instead, skip
undefined symbol.  Undefined symbol will be reported by relocate_section.

	* elfxx-x86.c (elf_x86_size_or_finish_relative_reloc): Skip
	undefined symbol in finishing phase.

(cherry picked from commit a9e61227)