as they unnecessarily clutter the diagnostic output and make the
experience of adding a new target to the compiler more painful than
it should be.
target_os and target_arch are still being tested in the
well-known-values.rs test, but in one place.
This is fixed by simply using the currently registered target in the
current session. We need to use it because of target json that are not
by design included in the rustc list of targets.
