Add ALMA9 and install libc++ on debian-clang

• Add ALMA9 with clang v16 and LLD linker (not via CVMFS)
• Add libc++ (LLVM C++ STL) to debian-clang

The idea is to build in both debian-clang and alma9-cern with LLVM's libcxx to ensure we can build both standard libraries. Requires adding -stdlib=libc++ to the compile arguments (maybe meson cpp_stdlib=libc++ works as well, needs testing).