copy the correct version of LLVM into the stage0 sysroot In some cases(see https://github.com/rust-lang/rust/issues/109314), when the stage0 compiler relies on more recent version of LLVM than the beta compiler, it may not be able to locate the correct LLVM in the sysroot. This situation typically occurs when we upgrade LLVM version while the beta compiler continues to use an older version. Fixes #109314