libc: implement gai_strerror

Code from musl.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
pthread: implement raise() function

The man page for raise says:

In a single-threaded program it is equivalent to

           kill(getpid(), sig);

In a multithreaded program it is equivalent to

           pthread_kill(pthread_self(), sig);

In our case we should mimic the second. It is a good question then whether this
function should be in a more generic place or in pthread.cc itself, but I will
argue for the second, since it will make it easier for people to notice that
this is what our implementation does.

Of course at this moment pthread_kill is stubbed and so is raise. But if we
ever implement the first, we gain the later for free.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>