Add closure example for wasm_cc_binary (#941)

* Stub out addition of closure_externs_file option in wasm_cc_binary

* WIP

* args are passed, externs cannot be found

* try location

* update tests

bazel/test_external/hello-embind-externs.js

@@ -0,0 +1,2 @@
+// This file prevents customJSFunctionToTestClosure from being minified by the Closure compiler.
+Module.customJSFunctionToTestClosure = function() {}