[libomptarget] Build a minimal deviceRTL for amdgcn

Summary:
[libomptarget] Build a minimal deviceRTL for amdgcn

Repeat of D70414, with an include path fixed. Diff for sanity checking.

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: ABataev, jdoerfert

Reviewed By: ABataev

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

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D70971