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

[Matheus Izvekov via llvm-commits]

mizvekov wrote:

I understand the concern about decoupling the runtimes build from the main llvm build.

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.

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