Fix numeric abbreviation for --disable-float8-byval.

When committing abd94bca, I tried to make
it decide what kind of abbreviation to use based only on SIZEOF_DATUM,
without regard to USE_FLOAT8_BYVAL.  That attempt was a few bricks short
of a load, so try to fix it, and add a comment explaining what we're
about.

Patch by me; review (but not a full endorsement) by Andrew Gierth.