build: support 64 bits darwin builds

Before this commit, gyp built 32 bits versions of libuv exclusively.

Enable with:

  $ ./gyp_uv -Dtarget_arch=x64
darwin: implement uv_set_process_title, part 1

Apply undocumented Carbon magic to change the process title in a way
that's visible in the Activity Monitor.
darwin: implement uv_set_process_title, part 2

Make changes to the process title visible to tools like `ps`.

The argv clobber technique is reasonably portable across Unices;
the common code has been moved into src/unix/proctitle.c and is used
on Linux and OS X. Other platforms will probably follow in the future.