elf_backend_archive_symbol_lookup

elf_backend_archive_symbol_lookup might be called when the linker hash
table has entries of type generic_link_hash_entry.  This happens for
instance when running the mmix target linker testsuite where the
output is mmo but input is elf64-mmix.

	* elf-bfd.h (struct elf_backend_data): Return bfd_link_hash_entry*
	from elf_backend_archive_symbol_lookup.
	(_bfd_elf_archive_symbol_lookup): Return bfd_link_hash_entry*.
	* elf64-ppc.c (ppc64_elf_archive_symbol_lookup): Likewise.  Check
	we have a ppc_hash_table before accessing ppc_link_hash_entry
	fields.
	* elflink.c (_bfd_elf_archive_symbol_lookup): Return
	bfd_link_hash_entry*.
	(elf_link_add_archive_symbols): Adjust to suit.