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.

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -246,6 +246,9 @@ const WASM_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
     ("simd128", None),
     ("atomics", Some(sym::wasm_target_feature)),
     ("nontrapping-fptoint", Some(sym::wasm_target_feature)),
+    ("bulk-memory", Some(sym::wasm_target_feature)),
+    ("mutable-globals", Some(sym::wasm_target_feature)),
+    ("reference-types", Some(sym::wasm_target_feature)),
 ];
 
 const BPF_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[("alu32", Some(sym::bpf_target_feature))];