wasm: don't block //go:wasmexport because of running goroutines

This fixes bug https://github.com/tinygo-org/tinygo/issues/4874.

src/runtime/scheduler_none.go

@@ -12,6 +12,9 @@ const hasParallelism = false
 // Set to true after main.main returns.
 var mainExited bool
 
+// dummy flag, not used without scheduler
+var schedulerExit bool
+
 // run is called by the program entry point to execute the go program.
 // With the "none" scheduler, init and the main function are invoked directly.
 func run() {