[libc] [llvm] [libc] delete hdrgen (PR #117220)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

Maybe we can just delete the section https://libc.llvm.org/gpu/building.html#standalone-cross-build and just support https://libc.llvm.org/gpu/building.html#runtimes-cross-build ? Then there are no more references to libc in DLLVM_ENABLE_PROJECTS (which is the goal of #78479)?

> This method is generally not recommended as it can only target a single GPU architecture.

In which case, I wonder if we can just drop support for that?

https://github.com/llvm/llvm-project/pull/117220