build: enable handling of alpine-linux-musl triple
Currently, when building LLVM/clang on Alpine Linux, CMake toolchain file specifies incorrect `<cpu_arch>-unknown-linux-musl` target, which makes the build immediately fail. Correct target that allows building on Alpine should be specified as `<cpu_arch>-alpine-linux-musl`, with `<cpu_arch>` replaced with respective CPU platform, e.g. `aarch64`.
