initial fixes for building kj/main.c++ utilities on mingw

Note, this isn't all that's needed to fix the mingw build (cross compiling from linux).  In particular, this only fixes what I needed to use kj/main.c++ (and dependencies).

I'm assuming that the desired way of dealing with utf-16 windows APIs is to keep everything internally as utf-8 (as advocated by http://utf8everywhere.org/).

With wider windows API usage, we'll eventually want to extract a common way of doing the utf-8 <-> utf-16 translation, rather than calling MultiByteToWideChar all over the place.  I'm not sure what that will look like.
I think this is only true if the file descriptor happens to be wrapping a console device, or otherwise has had its mode set such that the C runtime knows to translate to wide characters. It's possible that the file descriptor has been redirected to a file, and we may expect the file to come out as ASCII in that case.

I think the right thing to do here is to stick with the write() call rather than WriteFile() and let the C runtime decide what translation to do. It's fine to do two write()s here -- I got a bit carried away with my optimization. :)

BTW I believe Win32 actually can write from multiple buffers -- `WriteFileGather()` does this. But it's not really necessary here.