Make it easier to handle detected problems by providing the function
signature(s) involved in cases of missing argument extensions.

For the purpose of verifying proper arguments extensions per the target's ABI,
introduce the NoExt attribute that may be used by a target when neither sign-
or zeroextension is required (e.g. with a struct in register). The purpose of
doing so is to be able to verify that there is always one of these attributes
present and by this detecting cases where sign/zero extension is actually
missing.

As a first step, this patch has the verification step done for the SystemZ
backend only, but left off by default until all known issues have been
addressed.

Other targets/front-ends can now also add NoExt attribute where needed and do
this check in the backend.