Fix aarch64-linux-android target installs

Problem you are trying to solve

Getting android targets to build out of the box.

Solution you'd like

Auto setup some variable so that cargo can link properly for android and cc crate works properly.

For linux the only variable needed is (assuming android NDK installer properly setup its environment):

export CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER=aarch64-linux-android21-clang

If the NDK installer was defective another variable is needed:

export PATH="$PATH:ANDROID_HOME/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64/bin/"

Notes

Without CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER it is impossible to build any rust project for android whether or not this project uses some "-sys" libraries, rendering the default rustup install for android useless.

EDIT: In case you are wondering aarch64-linux-android21-clang is chosen as it is the default chosen for cc crate as well and having different API level linkers may be bad.

Start by locating rustup's Android target installation and environment setup entry points; the issue does not name specific files or tests. Reproduce an aarch64-linux-android build, check how the linker and NDK PATH are configured, and verify that a default installation can build without manual environment variables.