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

[Michał Górny via cfe-commits]

mgorny wrote:

> But `lib` is used for 32-bit libraries, and `lib64` for 64-bit libraries.

…and just to be clear, Flang runtimes are installed into plain system `CMAKE_INSTALL_LIBDIR` rather than `clang/lib/...` path (along with the "host" Flang libraries), and they don't have any arch-suffix, so different libdirs is the only way to distinguish 32-bit and 64-bit runtime.

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