Disable all standard lib functions for NVVM.

Summary:
NVVM doesn't have a standard library, as currently implemented, so this
just isn't going to work.  I'd like to revisit this, since it's hiding
opportunities for optimization, but correctness comes first.

Thank you to hfinkel for pointing me in the right direction here.

Reviewers: tra

Subscribers: echristo, jhen, llvm-commits, hfinkel

Differential Revision: http://reviews.llvm.org/D16604

llvm-svn: 258884