[clang] [RFC] [clang][Toolchain] Treat "pc"/"unknown" vendor interchangeable (PR #97802)

[Fangrui Song via cfe-commits]

MaskRay wrote:

How is Rust built? If the system compiler-rt libraries are installed to `/usr/lib/llvm-18/lib/clang/18/lib/x86_64-pc-linux-gnu`, which means that `LLVM_DEFAULT_TARGET_TRIPLE=x86_64-pc-linux-gnu`, the Rust build should also use x86_64-pc-linux-gnu.

If we don't consider Rust, Clang itself can find the library whether or not "pc" or "unknown" is used.

https://github.com/llvm/llvm-project/pull/97802