[flang] [llvm] [Premerge] Add flang-rt (PR #128678)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> The build failiures on the windows buildkite look relevant.

Indeed. I should not build on Windows for something that is not even tested on Windows. 

Btw, I also noticed that it adds LLVM_ENABLE_PROJECTS=llvm, even though `llvm` is not a valid argument for `LLVM_ENABLE_PROJECTS`. I left it in the result of `compute-projects-to-test` because I think it is what triggers `check-llvm`.

https://github.com/llvm/llvm-project/pull/128678