Set pg_am.amstrategies to zero for index AMs that don't have fixed

Set pg_am.amstrategies to zero for index AMs that don't have fixed
operator strategy numbers, ie, GiST and GIN.  This is almost cosmetic
enough to not need a catversion bump, but since the opr_sanity regression
test has to change in sync with the catalog entry, I figured I'd better
do one.