./make-standalone-toolchain.sh --toolchain=arm-linux-androideabi-4.9 --platform=android-21 --install-dir=/Users/xxx/Library/Android/sdk/toolchain/
报错：
Refusing to clobber existing install directory: /Users/xxx/Library/Android/sdk/toolchain/. make-standalone-toolchain.sh used to install a new toolchain into an existing directory. This is not desirable, as it will not clean up any stale files. If you wish to remove the install directory before creation, pass --force.