[llvm] [flang-rt] fix missing llvm_gtest target (PR #67936)

[via llvm-commits]

pscoro wrote:

> It's still does not feel right that we are copy pasting that invocation around, instead of splitting out these parts of LLVM main build which we still want to be used as a dependency on everything, runtimes included.

Fully understand. Unfortunately there is a lot of haphazard cmake code in llvm and I tried my best to work around what already exists without letting the scope of the patch creep too far (it had already creeped much further than the original intention), so in an effort to keep this patch manageable within my time constraints I could not go down that rabbit hole. I agree that all the projects that use similar gtest hotfixes should be revisited and this should be cleaned up.

https://github.com/llvm/llvm-project/pull/67936