[llvm] [CI] Add flang-rt to pre-merge configuration (PR #156039)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

LGTM. Thanks. I was meaning to put this back in yesterday.

This should go a lot better than the last time we tried to enable `lfnag-rt` now that we have fully migrated to the runtimes build and don't need to stuff `flang-rt` in with the libc++ testing.

https://github.com/llvm/llvm-project/pull/156039