[libomptarget] Implement target_impl for amdgcn

Summary:
[libomptarget] Implement target_impl for amdgcn

Smallest atomic addition for a new target. Implements enough of the amdgcn
specific code that some of the source files under nvptx/src could be compiled,
without modification, to run on amdgcn.

This foreshadows a work in progress patch to move said source out of nvptx/src.
Patch based on fork at https://github.com/ROCm-Developer-Tools/llvm-project

Reviewers: ABataev, jdoerfert, grokos, ronlieb

Subscribers: jvesely, jfb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D69718