[llvm] [clang] Hurd: Add x86_64 support (PR #78065)

[Fangrui Song via cfe-commits]

MaskRay wrote:

Thanks for the patch. It's in a very good shape and I did not need to make many nitpicky comments like I had to do for other "support a new target triple" driver patch:)

Could you add some descriptions?

https://github.com/llvm/llvm-project/pull/78065