[OpenMP][AMDGCN] Support OpenMP offloading for AMDGCN architecture - Part 1

Summary:
Allow AMDGCN as a GPU offloading target for OpenMP during compiler
invocation and allow setting CUDAMode for it.

Originally authored by Greg Rodgers (@gregrodgers).

Reviewers: ronlieb, yaxunl, b-sumner, scchan, JonChesterfield, jdoerfert, sameerds, msearles, hliao, arsenm

Reviewed By: sameerds

Subscribers: sstefan1, jvesely, wdng, arsenm, guansong, dexonsmith, cfe-commits, llvm-commits, gregrodgers

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D79754