Rollup merge of #131830 - hoodmane:emscripten-wasm-eh, r=workingjubilee

Add support for wasm exception handling to Emscripten target

This is a draft because we need some additional setting for the Emscripten target to select between the old exception handling and the new exception handling. I don't know how to add a setting like that, would appreciate advice from Rust folks. We could maybe choose to use the new exception handling if `Ctarget-feature=+exception-handling` is passed? I tried this but I get errors from llvm so I'm not doing it right.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -37,6 +37,7 @@ const GATED_CFGS: &[GatedCfg] = &[
     (sym::sanitizer_cfi_normalize_integers, sym::cfg_sanitizer_cfi, Features::cfg_sanitizer_cfi),
     // this is consistent with naming of the compiler flag it's for
     (sym::fmt_debug, sym::fmt_debug, Features::fmt_debug),
+    (sym::emscripten_wasm_eh, sym::cfg_emscripten_wasm_eh, Features::cfg_emscripten_wasm_eh),
 ];
 
 /// Find a gated cfg determined by the `pred`icate which is given the cfg's name.

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