Update gcc version to 4.9 everywhere. NDK r11 stopped support for gcc 4.8. Switch from bin/exe format to zip format. Drop support for linux/x86, since r11c no longer supports it. Change the NDK URLs from http://dl.google.com/android/ndk/ to https://dl.google.com/android/repository/. Add a flag, -ndkdir, to specify where the downloaded NDK files for processing are found and downloaded. Check the status code from the http.Get that fetches the NDK files. Change-Id: I836de6611bd04f5220e01937a3288a91604b8999 Reviewed-on: https://go-review.googlesource.com/23592 Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>