wasm: correctly return from run() in wasm_exec.js

Instead of hanging forever, it should return the exit code from os.Exit.

testdata/wasmexit.go

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"os"
+	"time"
+)
+
+func main() {
+	println("wasmexit test:", os.Args[1])
+	switch os.Args[1] {
+	case "normal":
+		return
+	case "exit-0":
+		os.Exit(0)
+	case "exit-0-sleep":
+		time.Sleep(time.Millisecond)
+		println("slept")
+		os.Exit(0)
+	case "exit-1":
+		os.Exit(1)
+	case "exit-1-sleep":
+		time.Sleep(time.Millisecond)
+		println("slept")
+		os.Exit(1)
+	}
+	println("unknown wasmexit test")
+}