mips64 elf64 reloc data structures

[Roland McGrath]

> I don't know of any such place.  But then, Uli suggested adding the
> structures to the installed elf.h would be nice for other applications
> that wanted to support the mips elf64 relocation format, even though
> the macros wouldn't be automatically correct for them.  I wouldn't
> mind moving all of the structures to some glibc-private, or even
> mips-specific headers, if it turned out to be possible.

With a cleaner version of the elf.h patch, I might be happy.  We'll see.
Using a private header is always an available safe route, and I am hesitant
to add to elf.h without being completely satisfied with the names and such.