[llvm] [libc] [libc] Fix libc-hdrgen crosscompiling (PR #78227)

[Petr Hosek via llvm-commits]

petrhosek wrote:

I suspect it's related to `-DLLVM_OPTIMIZED_TABLEGEN=ON` but I cannot reproduce this locally using your CMake options, was this with clean or incremental build?

https://github.com/llvm/llvm-project/pull/78227