fix emscripten now method

see https://github.com/emscripten-core/emscripten/blob/main/system/include/emscripten/emscripten.h#L85

src/wasm.rs

@@ -128,11 +128,17 @@ pub fn now() -> f64 {
 #[cfg(not(any(feature = "wasm-bindgen", feature = "stdweb")))]
 mod js {
     extern "C" {
+        #[cfg(not(target_os = "emscripten"))]
         pub fn now() -> f64;
+        #[cfg(target_os = "emscripten")]
+        pub fn _emscripten_get_now() -> f64;
     }
 }
 // Make the unsafe extern function "safe" so it can be called like the other 'now' functions
 #[cfg(not(any(feature = "wasm-bindgen", feature = "stdweb")))]
 pub fn now() -> f64 {
-    unsafe { js::now() }
+    #[cfg(not(target_os = "emscripten"))]
+    return unsafe { js::now() };
+    #[cfg(target_os = "emscripten")]
+    return unsafe { js::_emscripten_get_now() };
 }