128-bit integers on earlier versions of GCC

Looks like earlier versions of GCC (such as 4.4.7, which is the version installed on CentOS 6.4) don't support the `__int128` type:

```
[  1%] Building C object src/CMakeFiles/embedded_libcork.dir/libcork/cli/commands.c.o
In file included from /home/vagrant/git/libcork/src/../include/libcork/core/hash.h:18,
                 from /home/vagrant/git/libcork/src/../include/libcork/core.h:21,
                 from /home/vagrant/git/libcork/src/libcork/cli/commands.c:16:
/home/vagrant/git/libcork/src/../include/libcork/core/u128.h:31: error: expected ‘:’, ‘,’, ‘;’, ‘}’ or ‘__attribute__’ before ‘u128’
```