Update list of allowed aarch64 features I recently added these features to std_detect for aarch64 linux, pending [review](https://github.com/rust-lang/stdarch/pull/1146). I have commented any features not supported by LLVM 9, the current minimum version for Rust. Some (PAuth at least) were renamed between 9 & 12 and I've left them disabled. TME, however, is not in LLVM 9 but I've left it enabled. See https://github.com/rust-lang/stdarch/issues/993
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.