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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

Tested with a flang/ change here: https://github.com/llvm/llvm-project/pull/130955. Interestingly, it build flang even on Windows: https://buildkite.com/llvm-project/github-pull-requests/builds/156355#01958a65-0b87-4bbe-bc22-6eda010c2b94

Another test with a flang/ change without any change in .ci (https://github.com/llvm/llvm-project/pull/130960) reveals that it was always that way: https://buildkite.com/llvm-project/github-pull-requests/builds/156364#01958a85-e4f9-4be5-a217-dcfcbcfed301

Reason is what I already mentioned: `compute-projects-to-test()` always echo's the directory that has changed, with this PR it just falls back to the [`*)` case](https://github.com/llvm/llvm-project/blob/b2354ffc924289ac0cbf59d53b498c121da4bd22/.ci/compute-projects.sh#L61). Still merge? I don't intend to fix all the legacy problems.

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