[OpenMP][bugfix] Fix issues with C++ 17 compilation when handling math functions

Summary: In OpenMP device offloading we must ensure that unde C++ 17, the inclusion of cstdlib will works correctly.

Reviewers: ABataev, tra, jdoerfert, hfinkel, caomhin

Reviewed By: jdoerfert

Subscribers: Hahnfeld, guansong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61949

llvm-svn: 360804