[OpenMP] enable bc file compilation using the latest clang

Summary: adding cuda-rdc flag to allow extern global data

Reviewers: grokos

Reviewed By: grokos

Subscribers: gregrodgers, mgorny, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D44992

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@329072 91177308-0d34-0410-b5e6-96231b3b80d8