generate syscache info automatically

I want to report on my on-the-plane-to-PGCon project.

The idea was mentioned in [0]. genbki.pl already knows everything about
system catalog indexes. If we add a "please also make a syscache for
this one" flag to the catalog metadata, we can have genbki.pl produce
the tables in syscache.c and syscache.h automatically.

Aside from avoiding the cumbersome editing of those tables, I think this
layout is also conceptually cleaner, as you can more easily see which
system catalog indexes have syscaches and maybe ask questions about why
or why not.

As a possible follow-up, I have also started work on generating the
ObjectProperty structure in objectaddress.c. One of the things you need
for that is making genbki.pl aware of the syscache information. There
is some more work to be done there, but it's looking promising.

[0]:
https://www.postgresql.org/message-id/flat/CA%2BhUKGKdpDjKL2jgC-GpoL4DGZU1YPqnOFHbDqFkfRQcPaR5DQ%40mail.gmail.com