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

[Michał Górny via cfe-commits]

mgorny wrote:

I find this change quite confusing. It seems that `-static-libflangrt` now expects entirely different install location than `-shared-libflangrt`. Furthermore, the location used seems inconsistent with clang, i.e.:

    /usr/lib/llvm/21/bin/../../../../lib/clang/21/lib/x86_64-pc-linux-gnu/libflang_rt.runtime.a

whereas clang uses `lib/linux/libclang_rt.builtins-x86_64.a` and so on.

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