[OpenMP][libomptarget] Bringing up to spec with respect to OMP_TARGET_OFFLOAD env var
Right now, only the OMP_TARGET_OFFLOAD=DISABLED was implemented. Added support for the other MANDATORY and DEFAULT values.
Reviewers: gtbercea, ABataev, grokos, caomhin, Hahnfeld
Reviewed By: Hahnfeld
Subscribers: protze.joachim, gtbercea, AlexEichenberger, RaviNarayanaswamy, Hahnfeld, guansong, openmp-commits
Differential Revision: https://reviews.llvm.org/D50522