[llvm] Remember LLVM_ENABLE_LIBCXX setting in installed configuration (PR #134990)

[David Spickett via llvm-commits]

DavidSpickett wrote:

I've found the settings to get stage 1 clang to use the host compiler's libcxx:
```
  -DLLVM_RUNTIME_TARGETS=aarch64-unknown-linux-gnu \
  -DRUNTIMES_aarch64-unknown-linux-gnu_CMAKE_CXX_FLAGS="-I /usr/local/clang+llvm-19.1.7-aarch64-linux-gnu/include/c++/v1/ -I /usr/local/clang+llvm-19.1.7-aarch64-linux-gnu/include/aarch64-unknown-linux-gnu/c++/v1/ -L/usr/local/clang+llvm-19.1.7-aarch64-linux-gnu/lib/aarch64-unknown-linux-gnu/" \
```
Note that you have to set LLVM_RUNTIMES_TARGETS, or the pass through of options doesn't work, or at least, didn't seem to.
I also tried a single --sysroot but the llvm install package isn't in a layout that will work with that.

Needs some changes to be something I'm comfortable putting in a buildbot config, but at least I know it can work.

https://github.com/llvm/llvm-project/pull/134990