[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

llvm-svn: 323649