Add support for wasm exception handling to Emscripten target

Gated behind an unstable `-Z emscripten-wasm-eh` flag

compiler/rustc_feature/src/unstable.rs

@@ -202,6 +202,8 @@ declare_features! (
     (internal, allow_internal_unstable, "1.0.0", None),
     /// Allows using anonymous lifetimes in argument-position impl-trait.
     (unstable, anonymous_lifetime_in_impl_trait, "1.63.0", None),
+    /// Allows access to the emscripten_wasm_eh config, used by panic_unwind and unwind
+    (internal, cfg_emscripten_wasm_eh, "CURRENT_RUSTC_VERSION", None),
     /// Allows identifying the `compiler_builtins` crate.
     (internal, compiler_builtins, "1.13.0", None),
     /// Allows writing custom MIR