Fix new test case to work on (some?) big-endian architectures.

Fix new test case to work on (some?) big-endian architectures.

Use of pack("L") gets around the basic endian problem, but it doesn't
deal with the fact that the order of the bitfields within the struct
may differ.  This patch fixes it to work with gcc on NetBSD/macppc,
but I wonder whether that will be enough --- in principle, there
could be four different combinations of bitpatterns needed here.

Discussion: https://postgr.es/m/1650745.1679513221@sss.pgh.pa.us