misc/wasm: set non-empty default for os.Args

os.Args is usually never empty and the flag package panics if it is.
This commit makes os.Args default to ["js"] for js/wasm.

Change-Id: Iba527145686487b052da438fca40159e57e61a81
Reviewed-on: https://go-review.googlesource.com/117475Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/wasm/wasm_exec.js

@@ -51,7 +51,7 @@
 
 	global.Go = class {
 		constructor() {
-			this.argv = [];
+			this.argv = ["js"];
 			this.env = {};
 			this.exit = (code) => {
 				if (code !== 0) {