Also improve the build script for libffi, which is not used as part of the regular build.
(cherry picked from commit 969ae7f735)
Co-authored-by: Steve Dower <steve.dower@python.org>
Note that the support is not actually enabled yet, and so we won't be publishing these packages. However, for those who want to build it themselves (even by reusing the Azure Pipelines definition), it's now relatively easy to enable.
