wasm: use WASI ABI for exit function

This improves compatibility between the regular browser target
(-target=wasm) and the WASI target (-target=wasi). Specifically, it
allows running WASI tests like this:

    tinygo test -target=wasi encoding/base32

testdata/stdlib.go

@@ -19,4 +19,7 @@ func main() {
 	// package strings
 	fmt.Println("strings.IndexByte:", strings.IndexByte("asdf", 'd'))
 	fmt.Println("strings.Replace:", strings.Replace("An example string", " ", "-", -1))
+
+	// Exit the program normally.
+	os.Exit(0)
 }