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

[Fangrui Song via cfe-commits]

MaskRay wrote:

> > Could you add some descriptions?
> 
> What kind of description, where?

The commit message/first comment. Currently it's "No description provided."

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