[OpenMP] Customize CUDA-based tool chain selection

Summary: This patch provides a generic way of selecting CUDA based tool chains as host-device pairs.

Reviewers: arpith-jacob, caomhin, carlo.bertolli, ABataev, Hahnfeld, jlebar, hfinkel, tstellar

Reviewed By: Hahnfeld

Subscribers: rengolin, cfe-commits

Differential Revision: https://reviews.llvm.org/D29658

llvm-svn: 307271