[NFC][libomptarget] move remaining device specific code out of omptarget-nvptx.h

Summary:
[NFC][libomptarget] move remaining device specific code out of omptarget-nvptx.h

Strictly there is one remaining difference wrt amdgcn - parallelLevel is
volatile qualified on amdgcn and not on nvptx. Determining whether this is
correct - and how to represent the different semantics of 'volatile' under
various conditions - is beyond the scope of this code motion patch.

Reviewers: ABataev, jdoerfert, grokos

Subscribers: openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D69424