std: Update randomness implementation on Windows

This commit updates the OS random number generator on Windows to match the
upstream implementation in the `rand` crate. First proposed in
rust-lang-nursery/rand#111 this implementation uses a "private" API of
`RtlGenRandom`. Despite the [documentation][dox] indicating this is a private
function its widespread use in Chromium and Firefox as well as [comments] from
Microsoft internally indicates that it's highly unlikely to break.

Another motivation for switching this is to also attempt to make progress
on #44911. It may be the case that this function succeeds while the previous
implementation may fail in "weird" scenarios.

[dox]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa387694(v=vs.85).aspx
[comments]: https://github.com/rust-lang-nursery/rand/issues/111#issuecomment-316140155

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