[CUDA] Check that our CUDA install supports the requested architectures.

Summary:
Raise an error if you're using a CUDA installation that's too old for
the requested architectures.  In practice, this means that you need a
CUDA 8 install to compile for sm_6*.

Reviewers: tra

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D21869

llvm-svn: 274781