Converting the current bot config to use the CMake cache file that we
use in other bots (offload/cmake/caches/AMDGPUBot.cmake). This PR
removes all CMake settings that the cache file already sets and only
leaves those that were either not set explicitly or which differ.
Thus, first load the cache file and then adjust the settings to override
existing values.
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.
