misc/wasm: fix panic on os.Stdout.Sync() in the browser

Before this change running os.Stdout.Sync() in the browser would panic
the application with:

panic: syscall/js: Value.Call: property fsync is not a function, got undefined

Afterwards Sync() becomes a noop for compatibility reasons.

Change-Id: I1fcef694beb35fdee3173f87371e1ff233b15d32
Reviewed-on: https://go-review.googlesource.com/c/143138Reviewed-by: Richard Musiol <neelance@gmail.com>
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/wasm/wasm_exec.js

@@ -59,6 +59,9 @@
 				err.code = "ENOSYS";
 				callback(err);
 			},
+			fsync(fd, callback) {
+				callback(null);
+			},
 		};
 	}