posix: Add getcontext, setcontext, and makecontext

These functions are POSIX functions for saving
and restoring thread context. The are used by programs
written in Go language.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Zika L'Etrange <zika.letrange@gmail.com>
[ penberg: fix aarch64 build ]
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>