Enable Rust cross-compilation to Android

[svpolonsky]

Describe the changes

Modified build.rs scripts to pass ANDROID flags to cmake.

Reviewer, please voice your opinion if the following approach is OK:

// check if cross-compilation is required
let target = std::env::var("TARGET").unwrap();
if target.contains("android") {
    config.define("BUILD_FOR_ANDROID", "ON");
}

added ./scripts/cross-compilation/android directory

The directory has a README.md and a Dockerfile to build the required toolchain

[jeremyfelder]

Why isn't this in the ICICLE Cargo.toml? This seems like it will affect the users entire cargo setup