add __syscall symbol

An alias of syscall, __syscall is also a valid symbol. I am not using the
weak_alias macro because this is not part of musl.

The only tricky part here is to get the headers right. Our syscall.h file
includes the one in sys/, that comes from musl - we don't want to change that.
I change unistd.h then, where the definition of syscall already lives, and
include that one in the main syscall header.

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