sparc64 support for Solaris

[Joe Buck]

> > > Solaris 2.5.x does not save the 64-bit register state, so basically some
> > > registers will come back randomized (from other processes?).
> > 
> > hmm...
> > are you sure ?
> > Solaris2.5.1 ultra != solaris 2.5.1 32bit sparc.

I said:
> I am quite certain.

That is, I am quite certain that there is a problem.  I was not
certain (or correct) about the details.

Well, as has been pointed out, they are zero-extended, not random,
but the essense is that user-mode 64-bit code is unsafe.