[OpenMP][libomptarget] Bringing up to spec with respect to OMP_TARGET_OFFLOAD env var

Summary:
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

llvm-svn: 340542