Blind try to fix portability issue in commit 8f93bd85 et al.

The S/390 members of the buildfarm are showing failures indicating
that they're having trouble with the rint() calls I added yesterday.
There's no good reason for that, and I wonder if it is a compiler bug
similar to the one we worked around in d9476b83.  Try to fix it using
the same method as before, namely to store the result of rint() back
into a "double" variable rather than immediately converting to int64.
(This isn't entirely waving a dead chicken, since on machines with
wider-than-double float registers, the extra store forces a width
conversion.  I don't know if S/390 is like that, but it seems worth
trying.)

In passing, merge duplicate ereport() calls in float8_timestamptz().

Per buildfarm.