Upgrade Emscripten targets to use upstream LLVM backend

- Refactors the Emscripten target spec to share code with other wasm
   targets.
 - Replaces the incorrect wasm32 C call ABI with the old asmjs
   version, which is correct for both wasm32 and JS.
 - Updates the varargs ABI used by Emscripten and deletes the old one.
 - Removes the obsolete wasm32-experimental-emscripten target.
 - Temporarily makes Emscripten targets use panic=abort by default
   because supporting unwinding will require an LLVM patch.

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,
         _ => &[],
     }
 }