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

[Azat Khuzhin via cfe-commits]

azat wrote:

@MaskRay you still think that it does not worth to make this two targets (`x86_64-**pc**-linux-gnu` and `x86_64-**unknown**-linux-gnu`) interchangeable?

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