Auto merge of #43639 - TobiasSchaffner:master, r=alexcrichton

Add L4Re Support in librustc_back

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 https://github.com/rust-lang/libc/pull/591 to libc.

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

Thanks to humenda for reviewing.