[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