$(package)_download_file
No need to specify `$(package)_download_file` when it is equal to `$(package)_file_name`.
If we're cross-compiling for darwin on aarch64 hardware, we need need to use a Clang that will run on that hardware. Only tested in a Linux Docker container (aarch64-unknown-linux-gnu), running on an Apple M1 mac-mini (aarch64-apple-darwin20.5.0).
LLVM 8 has inherent nondeterminism in the compiler, fixed in LLVM 9+.