[OpenMP][libomptarget] Add global memory data sharing support for master-worker sharing.

Summary:
This patch adds support for the sharing of variables from the master thread of a team to the worker threads of the team.
The runtime uses a stack structure implemented as a doubly-linked list of slots with each slot having the exact same size as the size requested. This implementation leverages existing data structures. The runtime functions are added as separate functions to avoid interfering with the current interface. 

Limitations to be addressed in future patches:
- This current patch only employs global memory. In a future patch we will enable to usage for shared memory as an optimization.
- Allow the allocation of several requested sizes in the same slot.

Reviewers: ABataev, grokos, caomhin, carlo.bertolli

Reviewed By: grokos

Subscribers: Hahnfeld, guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D44260

llvm-svn: 327440