Convert the first AMDGPU buildbots to use the ScriptedBuilder introduced
llvm-zorg. For the motivation, see
https://github.com/llvm/llvm-zorg/pull/648.
Since the production buildbot still needs to be restarted for
ScriptedBuilder to work, only convert the builders that are currently in
staging for now. These are:
* openmp-offload-amdgpu-runtime
* openmp-offload-amdgpu-clang-flang
Both of them happen to be OpenMPBuilder.getOpenMPCMakeBuildFactory-based
builders before this change. They also set an environment variable that
the previous ScriptedBuilder did not, so we are adding support.
The corresponding llvm-zorg change is
https://github.com/llvm/llvm-zorg/pull/697.
