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

[tschwinge]

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.