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

[Joseph Huber via llvm-commits]

jhuber6 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?

I did a clean build and  still got
```
[1475/6997] Building native libc-hdrgen...
ninja: error: unknown target 'libc-hdrgen'
[1494/6997] Creating library symlink lib/libLLVMCodeGenTypes.so
FAILED: NATIVE/bin/libc-hdrgen /home/jhuber/Documents/llvm/llvm-project/build/NATIVE/bin/libc-hdrgen 
cd /home/jhuber/Documents/llvm/llvm-project/build/NATIVE && /usr/bin/cmake --build /home/jhuber/Documents/llvm/llvm-project/build/NATIVE --target libc-hdrgen --config Release
```
Curious that you can't reproduce it, here's a script that I used originally https://gist.github.com/jhuber6/c439286bf0da51905649b334efac02ee.

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