[OPENMP][NVPTX]Make runtime compatible with the original runtime.

Summary:
Reworked runtime to make it compatible with the requirements of the
original runtime library. Also, simplified some code to reduce number of
function calls.

Reviewers: gtbercea, kkwli0

Subscribers: guansong, jfb, caomhin, openmp-commits

Differential Revision: https://reviews.llvm.org/D55130

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@348003 91177308-0d34-0410-b5e6-96231b3b80d8