[llvm] [flang-rt] Update `test_flang` in CI to use `flang-rt` as one of the projects. (PR #135260)

[Daniel Chen via llvm-commits]

DanielCChen wrote:


> ```
> cmake -GNinja -DCMAKE_BUILD_TYPE=Release -D... ../runtimes
> ```
> 
> That should probably be a configuration `flang-rt` should support if it does not already.

Yes. `flang-rt` already supports this configuration. This is actually what we call "standalone" `flang-rt` build, as it doesn't need to build `flang`. 

However, in order to build `flang-rt` with this configuration, it will need a flang compiler that is built off the same github commit. This seems to me that the premerge build will need to build `flang` anyway.

Instead of doing the above, we could modify `monolithic-linux.sh` to do what we call "bootstrap" build as:
```
cmake -S "${MONOREPO_ROOT}"/llvm -B "${BUILD_DIR}" \
      -D LLVM_ENABLE_PROJECTS="${projects}" \
      -DLLVM_ENABLE_RUNTIMES="flang-rt" \
      ...
```
For both `flang` and `flang-rt` PRs. 

This configuration is what the buildbots are using too.

The difference is the addition of `-DLLVM_ENABLE_RUNTIMES="flang-rt" \`, which will put the `flang-rt` to the `resource-dir` that the driver is searching for. Without it, the `flang-rt` is still built at the moment as part of project `flang` but is put to the `DefaultLibPath` that driver is no longer searching as we will move away from it and disable building `flang-rt` as part of `flang` in the near future.


https://github.com/llvm/llvm-project/pull/135260