Bootstrap LLVM build

@juj [volunteered](https://github.com/WebAssembly/binaryen/pull/8657#issuecomment-4374447045) to add a bootstrapping step to build a recent LLVM/Clang using the system compiler and then use that clang to build Binaryen. This will unlock using recent C++ features (e.g. coroutines) in Binaryen while retaining the ability to build the full emsdk on older platforms where the system compiler does not support those features.