Make assertion failures visible

We didn't notice this, but disabling the debugging messages also disabled
the assertion failure messages! The user only sees "Aborted" and you
can only tell what happened with a debugger.

Instead of using kprintf(), pass the message directly to abort(), which
will print its argument to the console as expected.

Since we using the overloading abort(...), it now needs to be done in a
C++ source file. So move this function from libc/exit/assert.c, to
runtime.cc.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>