uv: Support android libuv standalone build

Tested most of my compilation in the previous patch on NodeJS
and extracted the patches from there. This patch ensures libuv
will be capable of building standalone as well, both with gyp
and Makefiles.

Build documentation was added to the README.md file.

Some tests are failing, and I have not heavily investigated
the reasons. The failures are generally on errors, and are
likely related to differences between fully POSIX-compatible
systems and android.