Delete powerpc64 symbol weakref field

Delete powerpc64 symbol weakref field

Now that u.alias is circular, weakref just duplicates its function.
Also, function symbols shouldn't be on the alias list so there is no
need to use alias_readonly_dynrelocs with them.

	* elf64-ppc.c (struct ppc_link_hash_entry): Delete weakref field.
	(ppc64_elf_copy_indirect_symbol): Don't set weakref.
	(alias_readonly_dynrelocs): Use u.alias rather than weakref.
	(ppc64_elf_adjust_dynamic_symbol): Don't use
	alias_readonly_dynrelocs for function symbols.