It seems like somebody used a dartboard while choosing integer widths
for the various values taken and returned by these functions ... and
then threw a fresh set of darts while writing the SQL declarations.

This patch brings the C code into line with what the SQL declarations
say, which is enough to make it not dump core on the particular 32-bit
machine I'm testing on.  But I think we could do with another round
of looking at what the datum widths *should* be.  For instance, it's
not all that sensible that hash_bitmap_info decided to use int64 to
represent a BlockNumber input when get_raw_page doesn't do it that way.

There's also a remaining problem that the expected outputs from the
test script are platform-dependent, but I'll leave that issue for
somebody else.

Per buildfarm.