Auto merge of #37702 - redox-os:redox, r=brson

Redox Support Preview

# Important - This is only a preview of a working `sys::redox`.

Compiling the Redox default distribution with this `libstd` results in a fully functioning distribution. As such, all further changes would be cosmetic or implementing features that have not been used by the default distribution (of which there are only a small number).

I do not expect this to be merged, but would like to discuss how it may be improved and get feedback.

There are a few `unimplemented!()` - `cloexec` for example. I have documented them below. These would be resolved before desiring a merge.

There are also issues with how the Redox syscall library is called - currently I am using a re-export in `libc` but that probably would not be desired.