[3.7] bpo-16575: Disabled checks for union types being passed by value. (GH-17960...

[3.7] bpo-16575: Disabled checks for union types being passed by value. (GH-17960) (GH-17970)

Although the underlying libffi issue remains open, adding these
checks have caused problems in third-party projects which are in
widespread use. See the issue for examples.

The corresponding tests have also been skipped.
(cherry picked from commit c12440c371025bea9c3bfb94945f006c486c2c01)