[libomptarget][nfc] Move omptarget-nvptx under common

Summary:
[libomptarget][nfc] Move omptarget-nvptx under common

Almost all files depend on require omptarget-nvptx, which no longer
contains any obviously architecture dependent code. Moving it under
common unblocks task/loop for amdgcn, and allows moving other code.

At some point there should probably be a widespread symbol renaming to
replace the nvptx string. I'd prefer to get things working first.

Building this (and task.cu, loop.cu) without a cuda library requires
some more refactoring, e.g. wrap threadfence(), use DEVICE macro more
consistently. Patches for that are orthogonal and will be posted shortly.

Reviewers: jdoerfert, ABataev, grokos

Reviewed By: ABataev

Subscribers: mgorny, fedor.sergeev, jfb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D71073