[clang] [flang] [driver] Generalize the code that adds the path of libflang_rt.runtime.a. (PR #134362)

[Michael Kruse via cfe-commits]

Meinersbur wrote:

This PR may allow us to build the shared library by default
https://github.com/llvm/llvm-project/blob/6c7c8b4776f51dc66138f71333e6ada0c33ca5e2/flang-rt/CMakeLists.txt#L159-L165

I would generally prefer if we could avoid having driver behavior depend on the existance of files, but in this case seems to mimic linker behavior anyway.

https://github.com/llvm/llvm-project/pull/134362