pg_catalog: fix up pg_catalog.pg_am table

[yuzefovich]

In postgres, the following query returns 0 rows:

-- Tables, matviews etc should have AMs of type 't'
SELECT pc.oid, pc.relname, pa.amname, pa.amtype
FROM pg_class as pc JOIN pg_am AS pa ON (pc.relam = pa.oid)
WHERE pc.relkind IN ('r', 't', 'm') and
    pa.amtype != 't';

while we receive a few hundred. We should fix it up (and adjust pg_regress roachtest accordingly).

Jira issue: CRDB-38491