Make flang-aarch64-out-of-tree build flang-rt

[Meinersbur]

Make FlangBuilder.getFlangOutOfTreeBuildFactory build the flang-rt runtime as an out-of-tree runtimes build with additional steps.

Fixes some issues with FlangBuilder too:

• Clears the build directories with the clean_obj flag set. Usually buildbot deletes the build directory whenever a CMakeLists.txt in a depends_on_projects subdirectory changes to get a clean build. Otherwise, an incompatiable CMakeCache.txt will carry on forever.
• Add flang and flang-rt as depends_on_projects. Otherwise, changes in these subdirectories do not trigger a build. Even worse, breaking commits will be attributed to the next commit in llvm/clang/mlir/openmp, as happed in e.g. https://lab.llvm.org/buildbot/#/builders/53/builds/11759 (commit causing failure was llvm/llvm-project@fe8b323)

This PR is not necessary for llvm/llvm-project#124126, it just wouldn't build the runtime anymore.

Affected builders:

• flang-aarch64-out-of-tree

Affected workers:

• linaro-flang-aarch64-out-of-tree

Verfied locally on x86_64.