wasm: support //go:wasmexport functions after a call to time.Sleep

This fixes a bug where `//go:wasmexport` functions would not be allowed
anymore after a call to `time.Sleep` (when using `-buildmode=default`).

testdata/wasmexport-noscheduler.go

@@ -1,5 +1,7 @@
 package main
 
+import "time"
+
 func init() {
 	println("called init")
 }
@@ -8,6 +10,10 @@ func init() {
 func callTestMain()
 
 func main() {
+	// Check that exported functions can still be called after calling
+	// time.Sleep.
+	time.Sleep(time.Millisecond)
+
 	// main.main is not used when using -buildmode=c-shared.
 	callTestMain()
 }