[OpenMP] Initial implementation of OpenMP offloading library - libomptarget device RTLs.

This patch implements the device runtime library whose interface is used in the code generation for OpenMP offloading devices.
Currently there is a single device RTL written in CUDA meant to CUDA enabled GPUs.
The interface is a variation of the kmpc interface that includes some extra calls to do thread and storage management that only make sense for a GPU target.

Differential revision: https://reviews.llvm.org/D14254



git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@323649 91177308-0d34-0410-b5e6-96231b3b80d8