libc: sys_errlist and sys_nerr symbols

Add sys_errlist and sys_nerr symbols to OSv. They're deprecated but
apparently there are still applications around that use them.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Cc: Sergei Turukin <sergei.turukin@rackspace.com>
Cc: Constantine Peresypkin <pconstantine@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>