[libomptarget][nfc] Move some source into common from nvptx

Summary:
[libomptarget][nfc] Move some source into common from nvptx

Moves some source that compiles cleanly under amdgcn into a common subdirectory
Includes some non-trivial files and some headers. Keeps the cuda file extension.

The build systems for different architectures seem unlikely to have much in
common. The idea is therefore to set include paths such that files under
common/src compile as if they were under arch/src as the mechanism for sharing.
In particular, files under common/src need to be able to include target_impl.h.

The corresponding -Icommon is left out in favour of explicit includes on the
basis that the it makes it clearer which files under common are used by a given
architecture.

Reviewers: jdoerfert, ABataev, grokos

Reviewed By: ABataev

Subscribers: jfb, mgorny, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D70328