PPC: ensure_undef_dynamic on weak undef only in plt

It's slightly weird to have a call to a weak function not protected by
a test of that function being non-NULL, but the non-NULL test might be
covered by a test of another function.  For example:
  if (func1)
    {
      func1 ();
      func2 ();
    }
where func2 is known to exist if func1 exists.

	* elf32-ppc.c (allocate_dynrelocs): Call ensure_undef_dynamic for
	weak undefined symols that only appear on PLT relocs.
	* elf64-ppc.c (allocate_dynrelocs): Likewise.