Add modifications needed for L4re in libstd

Add modifications needed for L4re in libstd

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:
c151220a84e40b65e45308cc0f3bbea4466d3acf

In many aspects implementations for linux also apply for the L4Re
microkernel.

Two uncommon characteristics had to be resolved:
* L4Re has no network funktionality
* L4Re has a maximum stacksize of 1Mb for threads

Co-authored-by: Sebastian Humenda <sebastian.humenda@tu-dresden.de>