bfd/ELF/x86: avoid layering violation in link hash table entry init

There's no reason not to do as the comment says, just like all other
architectures do when they need custom field: Call the allocation method
of the "superclass". Which is the ELF one, of which in turn the BFD one
is the "superclass", dealt with accordingly by
_bfd_elf_link_hash_newfunc().