Add the libstd-modifications needed for the L4Re target
This commit adds the needed modifications to compile the std crate for the L4 Runtime environment (L4Re).
A target for the L4Re was introduced in commit: c151220a84
In many aspects implementations for linux also apply for the L4Re microkernel.
Some uncommon characteristics had to be resolved:
* L4Re has no network funktionality
* L4Re has a maximum stacksize of 1Mb for threads
* L4Re has no uid or gid
Co-authored-by: Sebastian Humenda <sebastian.humenda@tu-dresden.de>