Create AVG() aggregates for int8 and NUMERIC which do not compute X^2,

src/backend/utils/adt/numeric.c

src/include/catalog/catversion.h

src/include/catalog/pg_aggregate.h

src/include/catalog/pg_proc.h

src/include/utils/builtins.h

src/test/regress/expected/create_aggregate.out

src/test/regress/sql/create_aggregate.sql