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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> I'm assuming you've tested the latest version somewhere?

Using the pre-merge itself, which seems to be more accurate than my local testing: https://buildkite.com/llvm-project/github-pull-requests/builds/154307#01956d9d-f62e-4104-b9bd-ca2c5b551686

This only tests if there is a change in the flang-rt/ directory. If you think it is necessary to also test various combinations of tests in other directories, please let me know.

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