Minimal liblibc for Redox

[jackpot51]

This implements a minimal liblibc for Redox. Previously, this was talked about here: https://internals.rust-lang.org/t/redox-support-in-liblibc-and-libstd/3954

Merging this would allow Redox to port more Rust software, and will also allow libstd to be ported to Redox.

Changes to src/lib.rs are minimal, and the src/redox.rs file is also minimal but may be expanded in the future.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[jackpot51]

This is in preperation for a much larger change in Rust's libstd, which can be seen in progress here: rust-lang/rust@master...redox-os:redox

[jackpot51]

@alexcrichton How does this look?

[jackpot51]

@alexcrichton Can a new cargo version be published?