[libc] [llvm] [libc] Fix the GPU build when building inside the NATIVE project (PR #118573)

[Nick Desaulniers via llvm-commits]

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

Thanks for figuring this out!

Looks like there's some more references to LIBC_HDRGEN_EXE in llvm/cmake/modules/CrossCompile.cmake that can get cleaned up, too.

https://github.com/llvm/llvm-project/pull/118573