pg_catalog: fix up `pg_catalog.pg_am` table

In postgres, the following query returns 0 rows:
```sql
-- 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

Epic CRDB-60809