std: Small cleanup and test improvement

This weeds out a bunch of warnings building stdtest on windows, and it also adds
a check! macro to the io::fs tests to help diagnose errors that are cropping up
on windows platforms as well.

cc #12516
native: Improve windows file handling

This commit splits the file implementation into file_unix and file_win32. The
two implementations have diverged to the point that they share almost 0 code at
this point, so it's easier to maintain as separate files.

The other major change accompanied with this commit is that file::open is no
longer based on libc's open function on windows, but rather windows's CreateFile
function. This fixes dealing with binary files on windows (test added in
previous commit).

This also changes the read/write functions to use ReadFile and WriteFile instead
of libc's read/write.

Closes #12406
rustc: Use libnative for the compiler

The compiler itself doesn't necessarily need any features of green threading
such as spawning tasks and lots of I/O, so libnative is slightly more
appropriate for rustc to use itself.

This should also help the rusti bot which is currently incompatible with libuv.
native: Recognize EISDIR

This recognizes the EISDIR error code on both windows and unix platforms to
provide a more descriptive error condition.