[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:

> > so it uses bootstrap build instead of part of the flang build
> 
> So adding `flang-rt` to `LLVM_ENABLE_PROJECTS` will cause it to use a just-built clang/flang (although a cursory glance seems to show there is no Fortran code in `flang-rt`?) for compilation? That is completely different from how all the other runtimes work (they use the host compiler for `LLVM_ENABLE_PROJECTS` and will use a just built compiler when specified in `LLVM_ENABLE_RUNTIMES`).

Thanks for the comment!

For building `flang`, if you only do `-DLLVM_ENABLE_PROJECTS=clang;mlir;flang`, it builds `flang` as well as `flang-rt` together. 
If you add `-DLLVM_ENABLE_RUNTIMES=flang-rt`, it will separate the build of `flang-rt` and publish it to `resource-dir`. 

The intention of this PR is to prevent building `flang-rt` as part of the `flang` build. Rather, build it with `LLVM_ENABLE_RUNTIMES`. This is how the buildbot is doing. 

As the `flang-rt` build is similar to `compiler-rt`, I mimicked `compiler-rt` in this PR. I am not sure if I missed anything or overdid anything.




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