Missing NDK support for static runtimes/no runtime

Created by: mtak-

There's no way to specify static or shared runtime (always shared) in the [ndk] section of .buckconfig.

We currently rely on libc++; however, there's a bug in c++_shared where exceptions do not always work on arm devices. The solution is to use c++_static, but this is not possible using cxx_library rules as they are now. A fix for this would be much appreciated.