Reorganize build scripts for libuv.

- Create separate libs for each architecture. OS X doesn't need this
  (we just build a universal binary), but it will help Linux.
- Move the libuv build stuff into wren.mk where the actual dependency
  on the lib is.
- Download libuv to deps/ instead of build/. That way "make clean"
  doesn't blow it away.
- Don't redownload libuv unless needed.

test/benchmark/delta_blue.wren

@@ -708,4 +708,3 @@ for (i in 0...20) {
 
 IO.print(total)
 IO.print("elapsed: ", IO.clock - start)
-