CPU specific hacks never should go to `core`. This simplifies the hack
and moves it to `cpu/native` by providing a custom `sched.h` that adds
the workarounds for glibc and adds an `#include_next "sched.h"`.
It's a bit of a pity that RIOT's sched.h has a name conflict with
POSIX's sched.h - otherwise those hacks wouldn't really be needed.
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