[libomptarget] Build a minimal deviceRTL for amdgcn

Summary:
[libomptarget] Build a minimal deviceRTL for amdgcn

The CMakeLists.txt file is functionally identical to the one used in the aomp fork.
Whitespace changes were made based on nvptx/CMakeLists.txt, plus the
copyright notice updated to match (Greg was the original author so would
like his sign off on that here).

This change will build a small subset of the deviceRTL if an appropriate toolchain is
available, e.g. a local install of rocm. Support.h is moved from nvptx as a dependency
of debug.h.

Reviewers: jdoerfert, ABataev, grokos, ronlieb, gregrodgers

Reviewed By: jdoerfert

Subscribers: jfb, Hahnfeld, jvesely, mgorny, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D70414