nvptx-as should diagnose unexpected input instead of silently ignoring it

See the _Something strange.  Ignore it._ code in `nvptx-as.c`.

As raised in <http://mid.mail-archive.com/87fvacld85.fsf@kepler.schwinge.homeip.net>:

> I'm not sure if silently ignoring "strange" tokens is a good strategy?

Bernd agreed in <http://mid.mail-archive.com/54DB7353.7060308@codesourcery.com>:

> I'd rather fail if anything unexpected is seen.
