PATCH: Check __x86_64__ instead of __WORDSIZE in bits/setjmp.h

[Roland McGrath]

> > We need to be very sure this cannot perturb anyone's ABI,
> > e.g. mangled names and the like.
> >
> > I'm not aware of any such issue with __jmp_buf in particular, but
> > we must be circumspect.

The patch looks OK now as far as nits go.  But I want to let some other
people give opinions about the safety vs name mangling et al.

Thanks,
Roland