Fix some incorrect preprocessor tests in tuplesort specializations

697492434 added 3 new quicksort specialization functions for common
datatypes.

That commit was not very consistent in how it would determine if we're
compiling for 32-bit or 64-bit machines.  It would sometimes use
USE_FLOAT8_BYVAL and at other times check if SIZEOF_DATUM == 8.  This
could cause theoretical problems due to the way USE_FLOAT8_BYVAL is now
defined based on SIZEOF_VOID_P >= 8.  If pointers for some reason were
ever larger than 8-bytes then we'd end up doing 32-bit comparisons
mistakenly.  Let's just always check SIZEOF_DATUM >= 8.

It also seems that ssup_datum_signed_cmp is just never used on 32-bit
builds, so let's just ifdef that out to make sure we never accidentally
use that comparison function on such machines.  This also allows us to
ifdef out 1 of the 3 new specialization quicksort functions in 32-bit
builds which seems to shrink down the binary by over 4KB on my machine.

In passing, also add the missing DatumGetInt32() / DatumGetInt64() macros
in the comparison functions.

Discussion: https://postgr.es/m/CAApHDvqcQExRhtRa9hJrJB_5egs3SUfOcutP3m+3HO8A+fZTPA@mail.gmail.com
Reviewed-by: John Naylor

src/include/utils/sortsupport.h

@@ -379,7 +379,9 @@ ApplySortAbbrevFullComparator(Datum datum1, bool isNull1,
  * are eligible for faster sorting.
  */
 extern int ssup_datum_unsigned_cmp(Datum x, Datum y, SortSupport ssup);
+#if SIZEOF_DATUM >= 8
 extern int ssup_datum_signed_cmp(Datum x, Datum y, SortSupport ssup);
+#endif
 extern int ssup_datum_int32_cmp(Datum x, Datum y, SortSupport ssup);
 
 /* Other functions in utils/sort/sortsupport.c */