outdated android build procedure

[gakonst]

from CI

WARNING:__main__:make_standalone_toolchain.py is no longer necessary. The
$NDK/toolchains/llvm/prebuilt/linux-x86_64/bin directory contains target-specific scripts that perform
the same task. For example, instead of:

    $ python $NDK/build/tools/make_standalone_toolchain.py \
        --arch x86_64 --api 21 --install-dir toolchain
    $ toolchain/bin/clang++ src.cpp

Instead use:

    $ $NDK/toolchains/llvm/prebuilt/linux-x86_64/bin/x86_64-linux-android21-clang++ src.cpp