[tz] Undoing the effect of the new alike-since-1970 patch

[Paul Eggert]

On 6/8/21 1:56 AM, Stephen Colebourne via tz wrote:
> It is possible to automatically derive the data for group 1 from the
> data for group 2. The other way around is not possible.

That's OK, as I'm not proposing the other way around. I'm merely 
proposing a flag to generate either group-1-style or group-2-style data 
from what we have now. This should be doable relatively easily. (And if 
I'm wrong, we can revisit this technical issue of course.)