ndk-build genrule support

Add support for genrule type targets to ndk-build. These would allow the user to add rules for generating source files. Since prebuilt libraries are implemented as source files for `PREBUILT_*_LIBRARY` modules, these would also be a way for users to expose externally built libraries to ndk-build. Something like ndkports or autoconf could be used to drive the genrule, which would create the "prebuilt" library.

Start by examining ndk-build and how PREBUILT_*_LIBRARY modules are implemented. Compare the requested genrule target with the existing ndkports or autoconf workflows. Done means users can define generation rules and expose externally built libraries through ndk-build.

Written by the indexing model from the issue text.