Make sure you have the NDK toolchain in your PATH, i.e. add <NDK>/toolchains/llvm/prebuilt/<host-tag>/bin/ to your PATH and then run the build with one of the android cargo targets like this:
cargo build -vv --features=vendored-openssl --target=aarch64-linux-android
If you are on macOS and you have the Xcode Command Line Tools installed everything should work out of the box, both for iphone and iphonesim. You can start the build with:
cargo build -vv --features=vendored-openssl --target=aarch64-apple-ios
To cross-compile for Windows from Linux install mingw32 and/or mingw64 and compile for the relevant cargo target, like:
cargo build -vv --features=vendored-openssl --target=x86_64-pc-windows-gnu