Fixes for 32-bit in stat and stream

These are pretty straightforward, I think, but I began this as a PR mostly hoping Travis would build 32-bit. Looks like that's not the case, unfortunately.

The most important part of this is the impact on `filesize()` for 32-bit builds. Since libuv returns a 64-bit value on all platforms, we might as well make use of it. Otherwise 32-bit Julia builds can be needlessly confused about the size of big files.