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_span/src/symbol.rs

@@ -570,6 +570,7 @@ symbols! {
         cfg_autodiff_fallback,
         cfg_boolean_literals,
         cfg_doctest,
+        cfg_emscripten_wasm_eh,
         cfg_eval,
         cfg_fmt_debug,
         cfg_hide,
@@ -823,6 +824,7 @@ symbols! {
         emit_enum_variant_arg,
         emit_struct,
         emit_struct_field,
+        emscripten_wasm_eh,
         enable,
         encode,
         end,