Blind try to fix portability issue in commit 8f93bd851 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 d9476b838.  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.

src/backend/utils/adt/timestamp.c

@@ -1546,9 +1546,10 @@ make_interval(PG_FUNCTION_ARGS)
 	result->day = weeks * 7 + days;
 
 #ifdef HAVE_INT64_TIMESTAMP
+	secs = rint(secs * USECS_PER_SEC);
 	result->time = hours * ((int64) SECS_PER_HOUR * USECS_PER_SEC) +
 		mins * ((int64) SECS_PER_MINUTE * USECS_PER_SEC) +
-		(int64) rint(secs * USECS_PER_SEC);
+		(int64) secs;
 #else
 	result->time = hours * (double) SECS_PER_HOUR +
 		mins * (double) SECS_PER_MINUTE +