cmd/compile/internal/types2: make built-in to accept type sets with single underlying types

This generalizes make the same way copy was generalized and eliminates
a use of optype.

Change-Id: I8221abd53d77dde8ead47c0075c13fd2a3221642
Reviewed-on: https://go-review.googlesource.com/c/go/+/357776


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>