Add std::os::set_errno as a counterpart to std::os::errno

This pull request adds a `std::os::set_errno` function to allow rust to manipulate the thread's `errno` value.  In my particular case, I am needing to do this when interacting with a c-library I am wrapping.

Note I don't have rust building on a Windows platform currently so have been unable to test my implementation for `[cfg(windows)]`
I don't know what "natural word size" would be, and it's not a concept exposed by the Rust language. It only exposes pointer-size integers, which are often different than the size of the largest general purpose registers.