cmd/gogio: add support for Windows

Now, gogio can build the program for Windows, using the `-target
windows`.

It will build with `-H=windowsgui`, by default. Also, it can compile for
multiple platforms if specified using `-target` (e.g. `-target arm, 386,
amd64`), the executable will have the respective suffix (i.e.
`_386.exe`).

gogio will also attach (any) appicon.png as executable icon resource and
include some information about the file and supported operating system.

Signed-off-by: Inkeliz <inkeliz@inkeliz.com>

cmd/gogio/iosbuild.go

@@ -222,12 +222,8 @@ int main(int argc, char * argv[]) {
 	if err := ioutil.WriteFile(plistFile, []byte(infoPlist), 0660); err != nil {
 		return err
 	}
-	icon := *iconPath
-	if icon == "" {
-		icon = filepath.Join(bi.pkgDir, "appicon.png")
-	}
-	if _, err := os.Stat(icon); err == nil {
-		assetPlist, err := iosIcons(bi, tmpDir, app, icon)
+	if _, err := os.Stat(bi.iconPath); err == nil {
+		assetPlist, err := iosIcons(bi, tmpDir, app, bi.iconPath)
 		if err != nil {
 			return err
 		}