[OpenMP][libomptarget] Suppress C++ 11 related warnings when building libomptarget-nvptx bitcode library, by Doru Bercea.

Summary: Pass -std=c++11 flag to compiler to suppress C++ 11 related warnings when building NVPTX bitcode library.

Reviewers: ABataev, caomhin, Hahnfeld

Reviewed By: ABataev, Hahnfeld

Subscribers: jdoerfert, Hahnfeld, jholewinski, mgorny, guansong, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D55772