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

[Matheus Izvekov via llvm-commits]

mizvekov wrote:

I see that some projects are already doing this workaround though, so this doesn't seem like it will make things much worse in that regards. And it does gets us the buildbots working again, which is critical. So LARTM,

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