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

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

src/runtime/runtime_wasip2.go

@@ -52,6 +52,3 @@ func sleepTicks(d timeUnit) {
 func ticks() timeUnit {
 	return timeUnit(monotonicclock.Now())
 }
-
-func beforeExit() {
-}