Populate CUDA flags on FreeBSD too, as many other toolchains do.

Summary:
This allows `clang` to be used to compile CUDA programs. Compiled
simple helloworld.cu with this.

Reviewers: dim, emaste, tra, yaxunl, ABataev

Reviewed By: tra

Subscribers: dim, emaste, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69990