Upgrade Emscripten targets to use upstream LLVM backend

- Compatible with Emscripten 1.38.46-upstream or later upstream.
 - Refactors the Emscripten target spec to share code with other wasm
   targets.
 - Replaces the old incorrect wasm32 C call ABI with the correct one,
   preserving the old one as wasm32_bindgen_compat for wasm-bindgen
   compatibility.
 - Updates the varargs ABI used by Emscripten and deletes the old one.
 - Removes the obsolete wasm32-experimental-emscripten target.
 - Uses EMCC_CFLAGS on CI to avoid the timeout problems with #63649.

src/librustc_codegen_llvm/llvm_util.rs

@@ -257,8 +257,7 @@ pub fn target_feature_whitelist(sess: &Session)
         "hexagon" => HEXAGON_WHITELIST,
         "mips" | "mips64" => MIPS_WHITELIST,
         "powerpc" | "powerpc64" => POWERPC_WHITELIST,
-        // wasm32 on emscripten does not support these target features
-        "wasm32" if !sess.target.target.options.is_like_emscripten => WASM_WHITELIST,
+        "wasm32" => WASM_WHITELIST,
         _ => &[],
     }
 }