[clang] [clang] [Driver] Fix respecting libdir when locating flang runtime (PR #127345)

[Fangrui Song via cfe-commits]

MaskRay wrote:

CLANG_INSTALL_LIBDIR_BASENAME can be "lib" or "lib64". I think it should only be used for libraries outside of llvm-project, like how cuda/amdgpu libs are installed by system. For llvm-project installed libs, it's much better to stick with a uniform "lib", which is easy to test in clang/test/Driver `%clang -###` and avoid platform differences (the test will not break on a different installation just because CLANG_INSTALL_LIBDIR_BASENAME is different).

https://github.com/llvm/llvm-project/pull/127345