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

[Daniel Chen via cfe-commits]

DanielCChen wrote:

> I think the static library should be the default (like libclang_rt.builtins). Most uses will already use the static library unless they went into the effort of building the shared library. Then, flang-rt and flang are version-locked without a versioning scheme (`flang_rt.runtime.a.1.0.0`) in place. Deploying an executable that depends on such a shared library is not straigtforward.

Agreed to have the static library as default (so I think`-dynamic-libflang_rt` may be the option to add to change the default linking method if users decide to build and link to the shared `flang-rt`, which would be different from `-static-libgcc`).

I posted a question on the Flang `general` slack channel asking if `flang-rt` will guarantee the API compatibility, which will require a versioning scheme.

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