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

[Azat Khuzhin via cfe-commits]

azat wrote:

>If you specify the wrong --target= (avoid -target , deprecated since Clang 3.4), clangDriver will not find the compiler-rt library.
>This works as intended.

But isn't this odd, that you depends on `LLVM_DEFAULT_TARGET_TRIPLE`?

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