Add L4Re Support in librustc_back

[TobiasSchaffner]

Add experimental support for x86_64-unknown-l4re-uclibc target, which covers the L4 Runtime Environment.

This pull request contains the changes that have to be made to librustc_back. It follows the changes humenda made in pull request rust-lang/libc#591 to libc.

Next steps will be the modifications to the needed libraries. (libstd, liballoc_system, libpanic_abort, libunwind)

Thanks to humenda for reviewing.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @arielb1 (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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit c151220 has been approved by alexcrichton

[bors]

⌛ Testing commit c151220 with merge 49ded7e...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 49ded7e to master...