[libc] [llvm] [libc] Disable old headergen checks unless enabled (PR #104522)

[Maksim Levental via llvm-commits]

https://github.com/makslevental approved this pull request.

LGTM - looking forward to trying it out here https://github.com/Xilinx/llvm-aie/pull/155

https://github.com/llvm/llvm-project/pull/104522