provide a working 64-bit integer datatype.  As recently noted, we've been
broken on such platforms since early in the 8.4 development cycle.  Since
it took nearly two years for anyone to even notice, it seems that the
rationale for continuing to support such platforms has reached the point
of non-existence.  Rather than thrashing around to try to make it work
again, we'll just admit up front that this no longer works.

Back-patch to 8.4 since that branch is also broken.

We should go around to remove INT64_IS_BUSTED support, but just in HEAD,
so that seems like material for a separate commit.