[llvm] [CI][flang][OpenMP] Build OpenMP runtime mod files for flang tests (PR #206517)

[Tom Eccles via llvm-commits]

tblah wrote:

> What's the compile time cost if we just build the entire runtime rather than just the modules? The runtimes generally are pretty fast to build and I'd like to avoid the diff unk in `monolithic-linux.sh` if possible.

I haven't measured it but I suspect it would be small. OpenMP doesn't seem to take long on my machine. I didn't do that here because I assumed that it was deliberately not enabled.

https://github.com/llvm/llvm-project/pull/206517