mips64 n32 and n64 suport in elf/

[Roland McGrath]

> I don't see any way to do prelinking work with REL, anyway, since the
> addend takes up the very location where we could possibly store the
> prelinked addresses.

That is true.  That's why for REL-using platforms like i386 prelink
converts REL to RELA.  So the ultimately right thing is to have that
support for mips as well, and have elf_machine_rela support for mips.

But given that we don't have that support now, this change is fine.