[llvm] Remember LLVM_ENABLE_LIBCXX setting in installed configuration (PR #139712)

[Paul Osmialowski via llvm-commits]

pawosm-arm wrote:

I'm testing some changes to our build.sh script as proposed by @DavidTruby that do not require cherry-picking anything on main: https://github.com/arm/arm-toolchain/pull/336 promising results so far, I'm still trying to refine them a bit.


https://github.com/llvm/llvm-project/pull/139712