Auto merge of #1292 - hermitcore:master, r=gnzlbg

add HermitCore support even if it doesn't have a UNIX interface

Currently, we redefine the interface between the HermitCore kernel (https://hermitcore.org) and Rust’s standard library. In the future, it will not depend on a POSIX-compatible C library. Consequently, we add the support of HermitCore if the "unix" environment isn’t set. It will be great to integrate this patch because it would simplify our development. The classical interface is still supported and part of the subdirectory "unix".