95266530d5
Add some unstable target features for the wasm target codegen I was experimenting with cross-language LTO for the wasm target recently between Rust and C and found that C was injecting the `+mutable-globals` flag on all functions. When specifying the corresponding `-Ctarget-feature=+mutable-globals` feature to Rust it prints a warning about an unknown feature. I've added the `mutable-globals` feature plus another few I know of to the list of known features for wasm targets. These features all continue to be unstable to source code as they were before.
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.