32->64 bit cross compilers

[dewar@gnat.com]

<<I think it would be *nice*, but I don't think we "have to".  This never
really worked right due to config files that didn't properly check
the bounds of constants, mostly in the older machines.  So I don't
think it's so terrible if it still doesn't, especially since changes to
fix these things can easily cause instability.
>>

To me it is pretty important to be able to do Linux->Sparc 64 cross
compilation at least, since this (along with NT->Sparc 64) is a likely
desired combination.